You could go either way on his one.
If you are tiling and are going to do a design in the field of the tile then you should plan where the OTR goes and installthe tile first. Also the advantage of installing the tile first is that you won't have to contend with the OTR in the way. It is easy enough to dril through the tile I always lightly score the glazing and then drill the holes with a hammer drill. Keep the drill slow until you get into the tile a bit and even then the slower the better to not crack the tiles. Also STAY AWAY FRO THE TILE EDGES when drilling!!! If you get an edge with the bit this is what cracks the tile, drill the field of the tile! You will have all sorts of holes to choose from when choosing where to put the fasteners. If you tile after you will have that OTR in your way and if it is cost you are worried about then tile the area behind the OTR with cheap tiles but I don't think that the few wxtra tiles will put you in the poor house.
Also when installing the mounting bracket you only have to have one bolt in a stud and you will find that in the installation instructions but 2 are better. The bolts with the expanders will be more than sufficient to hold that mounting bracket up and the reality of these OTR is that the cabinets really hold the majority of the weight and the bracket holds it in place, kinda, lol. Hit at least one stud and two if you can.
Here is a pretty comrehensive article I wrote for another DIY site check it out and you might find something that will be of use... http://www.refurber.com/advice/120/H...nge-microwave/
So I would install my tile first and just be careful when drilling the tiles
Good luck and be safe!