I'm in town in March for a large-ish convention, new to the hobby, but want to book a K-session or two. How far in advance can I/should I book?

If by K-session you mean with one of the agencies.. same day. They get kinda ticked off when you try to book days in advance. They have plenty of availability. Just book in the AM.

I frequent K-girl agencies in my area and will be in Vegas later this spring.  Any quality agencies I should check out?  Also, I assume all appointments are incall or do they have outcall?  I've only done outcall to my hotel in LV.