Hi,
I'd be surprised if any appliance manufacturer ever advised you to drill a hole in their product.
Do you have the model number- it would be hard to say if it's safe or not without seeing the fridge, and taking a look at the parts catalog (which is probably available online) would be the absolute best way to tell.
Most modern fridges have all the important stuff at the back. Fridge-freezers tend to have a cooling 'box' all the way around the freezer compartment, and upright non-fridge freezers tend to have the cooling pipes built into the shelves.
If it's an old fridge and you're nervous you could drill the holes very carefully- probing around in the expanded foam as you go to see if you're going to hit any coolant lines. That way the only thing that you'd risk would be putting an extra hole in your fridge that you'd have to fill if you find a coolant line- not the end of the world.
I drilled 6 holes in the side of this non-freezer fridge on friday night with no problems:
Frigidaire FRU17B2JW 16.7 Cu. Ft. Large Upright All Refrigerator with 5 Door Bins: 4 Wire Shelves
Here's the result (all Micro Matic parts other than the soda tap and the handles):
Only expanded foam between the inner and outer steel walls. This fridge is way better than a pre-built kegerator for me as I can fit 6 Cornelius 5-gallon (1/6 Bbl) kegs in it.