Is it common to tile behind an OTR microwave?
I tile behind the ones I do.  First this takes little time.  The top couple inches don't have to be perfect.  I try to make everything look nice but in the grand scheme of things this is a few minutes of work and you already have the tiles, thinset, grout, and tools going so just do it.

Some issues I have come across not tiling:


The top row is almost always slightly slanted and the cuts can be hard to make look nice.
If you switch microwaves (the new OTR micros don't last long) and it doesn't reach the tiles what do you do?
If the microwave goes past the tile then your install can be slightly slanted back.  When you have tile behind there it is easy to jam the microwave in knowing you have a flat surface.
If you decide to take micro out and go with a hood you probably won't have enough leftovers to get all the way up.