This is one of those questions where 100 people will have 100 answers. The bottom line is, how much do you depend on the watch?
My friend was a commercial diver and he had his 1665 checked every year, all seals replaced, and a pressure check. He depended on his watch for his life (this was before dive computers) so he spared no expense. Even so, after a successful pressure check, the same month he had an extended sat dive then a simple down-and-back the next week. On the 2nd dive it flooded at 80' depth. Some time during the long decomp it got a bit of crud in the HEV and that left it vulnerable to flooding.
So even regular checks and seal replacements can't guarantee it's watertight. My divers all have domed plexi crystals and if I bang one against a door frame, it's liable to crack the plexi and compromise the seal.
If the only thing you're doing is removing the caseback, then inspect the o-ring before you reinstall it. Lube it well so there's no binding between the back / o-ring / case, and make sure you screw the back on to spec. Then you'll probably be just fine.
Probably.