make sure you one of the more reputable shops when you do this and also make sure the original tachs guts back in case you ever want to put it back stock later down the road. it's kinda funny i was checking out the classic cougar forum the other day and there were guys that were downright pissed that some shop replaced the stock guts with aftermarket stuff. i don't know what brand the shop used but these guys were upset because they replaced the original factory stuff with aftermarket. i was like ummm...ok? maybe these guys don't realize that on most of the factory tachs when they die the car won't run because the tach is an integral part of the ignition system and usually even a cheapie aftermaket tach is more accurate than the factory stuff or at least not any less accurate. maybe it's just becuase they didn't know that's what was going to happen and they weren't warned about it first, or maybe because the original stuff is so expensive and the shop (s) kept the original guts, which i'm assuming weren't working in the first place otherwise why would they need to be sent out for repair.
anyway, you may want to make sure to get the original mechanisms back just in case.
personally, im fine with the stock stuff since my cars are usually fairly stock or not too far from it but if i was ever to build a really high dollar car or even just an engine then the aftermarket stuff would be nice for piece of mind. i also build all my own engines for piece of mind but the stock gauges have never let me down and i never rev the engine to more than 6k anyway, so even if it's off by 500 rpm it'll be fine since i build the engines to handle that much.