Ask how many rear triangle cold settings your LBS guy has done. As long as he has some experience and the right alignment tools (it sounds like he has both, but double check), I'd say invest the $30. Two to four millimeters doesn't sound like much, but having the drop outs out of parallel is suboptimal, and putting a 130mm OLD hub in a 126mm spaced triangle puts the dropouts (and the RD hanger) a little bit out of parallel. Is it a big deal? Probably not. But for $30, I'd say go for the cold setting and remove all doubt.