The airlock activity is most likely off gassing of CO2 and not actually fermentation. If your gravity readings stay consistent for three consecutive days, then it is safe to bottle.
The off gassing is caused by the prior fermentation process. As the beer is fermenting, it is creating alcohol and CO2. Most of the CO2 goes out through the airlock, but some off it gets locked up in the trub, yeast and hops. The CO2 that is locked up will slowly degas out through the airlock, eventually leading to a still beer (still beer is beer with absolutely no carbonation). There is no reason to wait until the still beer stage to bottle.
I've read that most beer that is ready to bottle has about 1 volume of CO2 in it at bottling time. I'm not sure what assumption BeerSmith makes for the amount of CO2 volume at priming time. I always undercarbonate a little, because of the volume of CO2 already present in the beer when I prime it, just to be on the safe side. For example, if I bottle early when it is still degassing, but the gravity is consistent for 3 consecutive days, I'll prime with the least amount of corn sugar for the style. However, if it is a beer that has been sitting for a long time and is obviously degassed, then I'll shoot for the middle range for the amount of priming sugar. I never put in the maximum (high end of the range) amount of priming sugar. By not using the maximum amount of priming sugar, I vastly reduce the risk of creating bottle bombs.