Here's a detailed discussion of this issue:
Park Tool Co. » ParkTool Blog » Chain Installation - Derailleur Bikes
Older (i.e. pre-8-speed) chains don't have any issues that I know of being split and then reassembled with a chain tool. However newer chains definitely do. Note that Shimano chain pins have mushroomed heads that are destroyed whenever you use a chain tool to break the chain and you must use a special rivet when you replace these. If you reuse the regular link pin then I think you're at high risk of breaking the chain on the road. I don't think SRAM chains have the same type of chain pins but I avoid splitting them with a chain tool and since all SRAM chains come with a master link this seems to strongly suggest that you should always split them using that rather than a chain tool. I did break a SRAM chain while riding about a year ago and I had been messing with lengthening/shortening the chain using a chain tool, so I'm guessing that the chain tool may have resulted in a damaged link pin.
I know for sure that the manufacturers of really narrow chains (10 speed etc) say to never reuse link pins.
My policy these days is to always use a master link to separate or lengthen the chain.