I always use the tap. You really want to minimise the number of times you open up the fermenter as it can lose some of the protective CO2 layer on top and can let wild yeast or other nasties in. Dipping things into your beer is another thing you want to minimise to reduce disturbing the beer and the risk of infection.
But if you open the tap slowly and just allow a small flow of beer, you can check gravity with virtually no disturbance and no risk of infection. The only thing to watch is that your airlock isn't overfilled with water because when you open the tap, it will bubble in reverse and you don't want any of that airlock liquid splashing into the fermenter. Some people put vodka in the airlock which would probably be sterile.
If it's early in the fermentation it's okay to disturb it a little. Get a friend to help you gently move the fermenter onto a milk crate or something to raise it up and give you access to the tap. If it's late in fermentation I wouldn't move it.