Well, it's not a hard and fast rule, but it is strongly suggested. If the provider is reviewed, there's a better than even chance that she's as advertised; the more reviews, the better. If there are no reviews, you're taking the chance of a scam or a bait-and-switch.
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and search for what you want.  There are incredible ladies on here in terms of looks and performance.  If you go with just an ad, first you may not get the girl in the ad or she may not do anything you really want.  It's worth the investment.

One of the best times I have ever had was with a unreviewed lady however she was with a Agency that had many good reviews on most of their other ladies..
With my $$$ I would be wary of unreviewed indies. However I am not a gambler..If I played Black Jack it would be a business

I follow this "rule", and think its a really good one. I broke it once and regretted the outcome.

I suspect that seeing a well-reviewed SP, who knows its your first hobby experience, will bring you the closest you can get to a guarantee that it will be a great one. Doing my "research" worked out better than I had dreamed for my first time!

experience.  I'm still a newbie, more or less, since it's been about a year since my first foray into the hobby.  A summary in order of "intense experience"

#1 - Has 4 reviews, but the most recent is 3 years old.  She now UTR.

#2 - No reviews, saw her ad on Eros, posted a 411 request and got a single, but very positive PM here.  She has a no review policy.

#3 - One review.  Seen her twice, and I'll be posting a review soon... or trying to submit one at her request.

#4 - Many reviews, but only as a FBSM provider, however, she's given me FS on more than one occasion.  I've not posted a review.

#5 - A FBSM provider, from a year ago, who retired, much to my dissappointment.

#6 - A very nice "non-pro" girl when I first started out via CL.  Don't use CL any more based on advice found here.

and last, while she doesn't merit a numerical desination, I've been trying to make a date with a very well reviewed provider (both in term of number of reviews and ratings) and two emails have gone without a reply.

And then there's a couple where I bailed, because something seemed wrong and I didn't like the vibe.

Ok, so if you're just getting in to this, yes, you probably want to stick with the well reviewed ladies.

However, sometimes you might want to walk on the wild side, if you're willing to gamble and lose.  Without reviews, it's really crapshoot.  But sometimes other avenues can provide a big benefit.

I'll give you an example, I saw an ad on CL for two ladies who have been working in my area.  They are not reviewed.  But they have been posting fairly frequently.  Tonight I was bored, so I thought I'd give them a try, knowing that it could turn out badly.  Luckily for me it turned out well.  They were fun, eager, not clock watchers, and very focused on making sure I had a good time.  Bottom line is I had an awesome time with two beautiful women, for what I would normally pay for 1 lady.

It could have just as easily worked out the other way, but if you don't gamble, you can't win big.

For your first few times, stick with a more sure thing... but as you progress, remember, just because they are not reviewed doesn't mean they won't provide you a quality experience

Well, it should be rule # 1 for a newbie.  CV especially is known for leaving ads up for ladies that are not even available anymore. Eros is what it is. It's always buyer beware.

Now, to be honest, I have found some great ladies, including one of my ATF's, who had no reviews on TER. It's a question of research, knowing what you are looking for and being firm about what you need to see from the lady in order to feel that she is legit. You also need to understand that not every non-reviewed lady will be a gem. Simply cut your loses and move on.

It takes a bit of doing to learn to use the search features for ladies located elsewhere...  I run individual searches for the name & phone.  Remember both can change!

Before I found TER, I had more poor experiences than successes.  One thing I have learned is the pro grade providers generally have web sites...  and pictures from more than 1 angle.  This increases the odds of the photo being real & currant.  Do NOT "Go Ape" over a single hot photo...  sadly, it may not even be real.  

The "2 call system" is standard...  variation from this is cause for suspicion.  

I'm not saying I'll never see an unreviewed provider...  but IF I choose to, I'll go in with both eyes open & then she WILL have at least one review.
skb

Rule #1 should be "This is Cyberspace anything can happen"

That said I verify,crossreference,check and recheck.

I am of the belief that a newbie guy should see well reviewed ladies and newbie ladies should only see guys with reviews.

Experience and understanding is a good thing!  Once you have been around long enough to have read about and heard many of the pitfalls and LE habits it will decrease your chances of making bad decissions.

If you go in scared you will make mistakes if you go in not afraid of anything you will make mistakes.  Mistakes for both sexes potentially affects, moeny, LE or bodily harm!  Be smart and be careful!