This may be a dumb question, but I was wondering how ladies prefer their cash.

Probably a good idea to limit the rolls of quarters to 1/2 the investment - just in case the lady needs change for the laundromat .

Lately my bank's ATMs have switched over to $50s, which is a trend I like.

Yep. $50s from ATM - no brainer.

The highest I've ever paid with $20s is $500.

Xoxo
Sheila

In the US, cash IS king.  AND, in the US, the standard is "paper money" = "greenbacks."  BUT also consider:
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1. Pre-1964 US dimes and quarters. Those coins are mostly (90%) silver.  A now retired Provider had a table on her rates page. As of July 2020, a roll of dimes, 50 @ 10 cents, has a face value of $5 but a silver (melt) value of ~$80.  A roll of quarters, 40 @ 25 cents, has a face value of $10 but a silver (melt) value of ~$165.  She also accepted other forms of precious metals and carried around a small scale (and, I guess, a small test kit to measure the purity (karats) of gold jewelry).  
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2. Gold Krugerrands, Gold Canadian Maple Leafs, Gold Australian Kangaroos, Chinese Gold Pandas, etc..  They come in 1 ounce and smaller denominations. If you don't want to carry around a stack of Benjamins, one 1 ounce gold coin is worth around $2000 today.  One 1/2 ounce gold coin is ~$1000.  Your date will have to trade it for US paper money at a dealer and pay a small premium to do so.
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3. Rolled coin. If you are a cheapskate and are OK with outcall to your location, try to pay your visitor with rolled coin. $500 in pennies is 1000 rolls of pennies and weighs around 275 pounds. She'll probably break the handle on her pocket book and give up after 40 pounds or less, which is less than $100. A bargain for you,  perhaps? I mention it but I don't recommend it. You could spend the rest of your life pinching those pennies instead of pinching pussie.

I've paid $2,000 for an hour with dollar bills. They like it when you make it rain and have to pick up the money off the floor.  

I like to pay with higher denominations, but that's not always available.  I recently paid $1,300 in 20s.  I didn't sense any disappointment.

in reality, I doubt there are many $500 providers washing at a coin-operated Laundromat.  

in $20's was three hundred.  My bank recently changed ATM machines as well and now I can get $20's, $50's or $100's with  any mix of the three denominations in $100 increments in a single withdrawal.

It got too embarrassing asking for change back.

After almost 20 years of hobbying, why would you ask such a dumb question.

but every once in a while you make me laugh "with" you. This is one of those times. lol