It is totally safe to pop the stopper off the carboy or crack the bucket lid from time to time. When using carboys, just be sure to sanitize the opening with a few sprays of Star San on a paper towel.
That said, you really shouldn't need to take a lot of gravity readings. I normally take 2 readings, one when I am running off into the carboys and one before I run off into the kegs.
If you have strong fermentation techniques, meaning you 1) pitch enough healthy yeast (for liquid yeast you must pitch starters - see http://www.mrmalty.com/calc/calc.html
for more info.), 2) aerate well (if you shake the carboy make sure you shake it off and on for about 20-30 minutes), 3) and control fermentation temps (never pitch warmer than 70 degrees, never let the fermentation get over 68-70 for most ales, never let the temp crash much more than 2 degrees in 24 hours, be aware that fermentation generates as much as 4-6+ degrees of heat over ambient at high krausen) - you usually should have no problem hotting your FG within 1 - 2 weeks. Temperature and amount of healthy yeast are extremely important, if you pitch too warm and the temp spikes and then crashes you could stall fermentation (not to mention off flavors from warm fermentation temp).
Also, in some cases some yeasts tend to flocculate and drop out early, especially English ale yeast. Often times this happens with about 4-6 points left to go. In this case you usually simply need to rouse the yeast with a sanitized racking cane and maybe bump the temp up a couple degrees, and usually it will finish.