[mainebrewer]

If your 'fridge has a black grill on the back, it probably does not have refrigerant lines in the sides. That's no guarantee.
Depending upon the age of the 'fridge, you may be able to google its brand, model, serial # and get a description of where the lines are located.
In my case I put my faucets on the side. My 'fridge did have the grill on the back and I was able to get a description of the line location.
However, I still used a holesaw to cut a small hole in the plastic interior, used a small screwdriver to probe around in the foam insulation between the plastic and the exterior metal skin to verify that there were no lines in the area.
I needed five holes for faucets and one for the CO2 line and this added an extra 30 minutes to the process.
I just didn't want to take a chance in turning my 'fridge into a very large  and expensive paperweight!

[oscarvan]

However, I still used a holesaw to cut a small hole in the plastic interior, used a small screwdriver to probe around in the foam insulation between the plastic and the exterior metal skin to verify that there were no lines in the area.

That's what I did for the CO2 lines in the side. There is no cost to being careful. The door I just when whammo!

[noisycricket]

Before you cut your fridge, go out and get a couple of kegs and a CO2 system.  Yes, use picnic taps first.  Get the hang of it and then decide if you want to dedicate a fridge to the task.  You can store the CO2 tank in the fridge for the time being.  I did that for a couple of years and then decided to cut the fridge up.

Just put a piece of plywood down (marine plywood is good) on top of the crispers to distribute the weight and you should be fine.

+1 on the plywood.  The top of the crispers is glass...
If you're pretty sure about dedicating a fridge to kegging, I would skip the picnic tap and get a real faucet.  Attach it right to the ball-lock connector and you're set.  Then later on, if you decide to drill a hole in the door, you already have the faucet.