Originally Posted by shauntraxler
On most occasions, yes, it's a bad thing.
Pre-pitch, oxygen in the wort is a good thing. It helps facilitate yeast reproduction. However, oxygen after fermentation has begun or ended is not usually a good thing. Oxygen entering the beer during the later stages of the process can lead to oxidation and off flavors.
Do your best not to disturb the brew mate.
Seeing as this is in the Wine forum, I have to disagree.
Most wineries stir or pump over their must/fermentation. With wine, you need to stir. Up until the last day or two of the ferment. Especially if you are fermenting with skins. If you don't punch down the cap (grapes) they will dry out, go VA, overheat the ferment, or oxidize the wine. If no skins are involved, stirring helps to keep the lees in suspension. This helps to clear the wine and to make sure the yeast are all working.
There are more reasons, but the short answer is Yes, please stir your must.