When drilling a tiled wall, should I aim for the seams or avoid them?
I would buy a set of glass bits.  I linked to some cheap ones - no idea if they are great or not but probably doesn't matter.  

It also doesn't matter if you do it on grout or tile - you need to anchor either way.

The key is to start with your smallest bit and work your way up to the hole size you need.  The tile breaks because of pressure.  By slowly increasing the hole size you are putting less force on it.