I’m of the opinion that the dregs are ok to taste, not to drink when I have lots of wines in the cellar at my disposal. One major rule of thumb for winemakers is to always, always, always, keep the carboys topped off. This reduces air in the head space.
I typically sample a carboy, filling half a glass of wine, at every racking. If the carboy has oak cubes in it, then I rack the wine into a clean carboy, and use a wine thief to collect my sample. I have heard that wine can stratify, so if you sample mid-rack, you are likely to get a skewed sample.
If the wine is relatively young or ready to bottle, therefore I am not looking to make any major changes, then I will pinch off the racking tube, then take a sample into a wine glass.