If the installation just requires drilling a hole in the tile for an anchor (like installing a slider for a handheld shower head or a towel bar), it is very easy to do it yourself. Not much worry of cracking tiles if you use the correct drill bit. If your item needs to be installed with thinset and grouted in place (like a ceramic soap dish) its not hard but you need to do when the tile is getting installed. Don't try and come back later and add this - its ten times the work.
IMO

Drilling into grout is extremely easy, drilling into marble is fairly easy, drilling into porcelain is a bit more difficult.

Depending on what I'm drilling, I'll use a carbide bit or a diamond bit.

Drill the holes, squirt in a blot of caulk, insert the anchor (if needed), and screw in whatever you're mounting.

The biggest risk in cracking a tile comes from tiles that are not fully backed with thinset, and you can add in excessive force used by the person on the drill (trying to push the bit into the tile instead of letting the abrasives do the cutting), and along with all that, thermal shock.

All that said...I've never cracked a tile when drilling.

Not sure what a handyman would charge for it, the caution there would be the handyman not having the best bits for the material being drilled through.

If your builder is on the hook, be there to assist with the layout if that'll make you feel better. He'd be liable for the whole shebang, in terms of R&Ring a tile if it were to crack. Have a level to make sure the bars are level, have a sharpie marker to mark the proposed screw locations on the tile.

If you brought in a 3rd party to do the drilling and they cracked a tile, it's possible a shady mechanic might blame poor tile installation as to why the tile cracked, taking himself off the hook for any repairs.

here is a vid on drilling once passed the commercial inro

http://www.diynetwork.com/videos/drilling-holes-in-ceramic-tile/15046.html

Thanks for all of your replies! Very helpful...

Posted by mongoct on Fri, Oct 2, 09 at 12:59 -- Drilling into grout is extremely easy, drilling into marble is fairly easy, drilling into porcelain is a bit more difficult.

So does this mean that I should try to install the screw holes into grout whenever possible?? I've been thinking about this; it will work for some items. However, my grout lines are all pretty thin, so putting a screw into grout may actually cut into the edges of TWO tiles rather than just a hole in one tile. WHICH IS PREFERABLE? Thanks.

my experieince with drilling tile- of all types- is simply getting it started! So if you start at a grout space, at least on one side of the towel bar, etc., that set of holes will be easier.
The tough ones are starting on a smooth super hard surface mid tile. We put tape over the tile to help a little and mark on top of the tape and drill straight through it.

Just remember, it's not the force applied, but the gentle pressure of the abrasive bit that cuts the tile. Force=cracked tile.