It all depends on whether you're comfortable with the screening procedures.  I was able to use multiple booker refs in SoCal for the first time I tried in Vegas many years ago, and then use the ref from the first Vegas agency for other agencies there until I was eventually set up with all of them.  My friends have told me that if you're in a hurry, they will want you to fax or text over your photo ID, and that even if you HAVE refs, if you want outcall to your hotel, they will want the photo ID.