I was looking at putting some wheel spacers on my Jeep TJ and was curious how safe they are?  Are they really worth it?  I don't want to put too much stress on everything else but I want the tires to stick out a bit and give me a wider stance.  Any opinions?  

There is some simple physics here. The further out you place wheels, the more leverage they have against axles and the more stress you place on them. You in effect weaken them. Generally this is not a problem in normal LD usage but it can accelerate wear and tear and cause a big problem sooner when exploring the edge of the "envelope".  

wheel spacers also cost nearly the same as decent steel wheels unless ya really like the wheels ya have. I won't touch em over 1". you'll find tire shops that wont put your tires back on for ya after a balance or tire repair also.

for the jeep 4.5" backspacing works and give ya the stance for tires up to 12.5"'s wide in the full range of articulation ya can get out of the stock and aftermarket short arms.

stock backspacing is 5.5 on all except the rubicon which is 5" backspacing.