Everyone varies. Mostly, not always, I like a degree of spontaneity. It does mean having plan b c d often but I don't like my diary (or my cock) to be at the beck and call of a prossie's 'appointment book.' Even planning in advance, there are prossies who find a TerriblySorrySomethingCroppedUp card, so at least a plan b is useful.
With hotlists, I have one for good possibilities and then I have an "on the day" one to move some of these to (they're available that day). I keep a separate hotlist of ones shagged (easy to remember which are good and which not) and a separate no-fucking-way one just to keep a few out of my hair that I definitely wish to avoid.
Agencies are too expensive for my tastes except when travelling. There are parlours within walking distance. I used to have one or two prossies on my list that were part time but knew me well enough and would always guarantee to see me, which was handy.