Its always good to prime before you
tile, as mentioned the moisture is sucked out too quickly and your wall tiles then become floor tiles, which is ok if you want a mosaic floor???
Lots of primers are available such as Ardex P51, BAL APD, Weber Hi-Flex, Bal-Bond etc.
Never prime if you are tiling over tiles, the adhesive will do the sticking job just fine but always use a good quality adhesive. A lot of tiler i know seem to find one they like or get on with and use it regardless, the point here is that there are hundreds of different types available and all for a specific purpose. They were invented for a reason!
If you go to Ardex.co.
uk and submit your details, they (free of charge) will send you a whopping great big folder full of everything your tiling heart desires. Even if you dont use thier products the info within tell you almost everything you need to know, what to use and how to prepare including all materials and specifications etc.