I guess if you're eventually going to open your fermentor anyway and let it sit wide-open to the air while you transfer your beer to kegs or bottling bucket, then doing a quick in-and-out to check gravity isn't going to matter much. I mean, sanitizing a thief isn't rocket science.
But if you're actively trying to minimize oxygen exposure (closed transfers during packaging, etc.) then opening the fermentor to check gravity is a poor choice, because you're undoing all of your effort in avoiding oxygen, and worst of all, it's just not necessary. If you must check FG, you can do it at packaging time.
At least for ales, if the following conditions are met, then you can pretty much take it to the bank that the beer is at FG:
- you can see evidence of krausen having risen and fallen
- you have half-decent yeast handling practices
- it has been at least 10 days** in a temperature controlled environment
** some yeasts are slower than others. The idea is you get to know how long it takes after using it a few times. If you really want to be careful, add a few more days, or weeks if you must. But there is no reason to leave beers sitting in fermentors for extended periods of time after they're done fermenting. All you're doing is occupying your equipment at that point, IMO. The beer can "condition" in the keg or bottle.
If the above conditions are met, and your FG check before packaging closely matches what you were expecting, then that should seal the deal.