It depends on how much they are being bent. If the adjustment is 2 degrees upright or flat from standard, your results should be fine. Adjusting more than that, Titleist will use heads that are already upright or flat before bending to avoid slight cosmetic markings to the chrome. Also, if you really want your irons to be factory perfect, order them. Even the best clubmaker with a great machine can leave slight marks on the irons. They really don't mean much after you've played or practiced a couple times. It just depends on how particular you are about cosmetics when you first get the irons.

Sometimes the loft and lie angle machines at the store differ in readings from place to place.   This depends on the accuracy of the machine and the operator.  I've seen variations of a degree in loft or lie angle even on the same lie angle machine.

I would advise doing it through Titleist because if they are bent at an independent golf shop the warranty will be voided. 

It'll vary from store to store. I would suggest having your local shop do it as they'll be bent on the same machine most likely by the same person every time you get them bent or checked. So the consistency will be there, but as others have stated the lofts will vary slightly from machine to machine and even from operator to operator on the same machine. 

Also as far as I know bending Irons doesn't void the warranty with Titleist so long as you stay with 2° of standard.

Having your irons bent by anyone other than Titleist will void your warranty.

I am currently playing Titleist DCI (gold) offset irons that I bought in June of 1994. Should I have someone look at them so see if they need to be adjusted or is that a given since they were purchased 18 years ago?

It can't hurt but those irons were cast steel so it would take a shock stronger than normal play or practice to alter the specs. If you're at the range or playing and the yardage gaps between clubs are OK, you probably have nothing to worry about. Those old DCIs are a nice, forgiving set.