I can't help with regard to the incall/outcall question - that is something you'll have to ask the ladies individually.
As to making the actual booking, I am sure that the ladies can accommodate both pre-bookings and "spur of the moment" bookings.
However, confirmed pre-bookings for tours are always preferable as it allows a lady to gauge the level of demand (and in turn determine whether it's worth her while) to tour to a particular location and helps to organise their time when they get there.
There have been many many times when ladies have had lots of enquiries/requests to tour to a particular city only to face no-shows and cancellations when they get there. This in turn leads to genuine mp.nets who wanted to see those ladies missing out because their preferred time slot was booked by someone else who didn't turn up.
So... bottom line is that it's preferable to pre-book but either way, make sure you follow-through, or give the lady plenty of warning if you have to cancel.