I always take it before I pitch so it doesn't interfere. That being said, I doubt the small amount of liquid into a batch of wort, or certainly if pitching dry yeast, doesn't really make a difference.
I take mine mid-transfer to the fermenting bucket from the brew kettle (once I chilled it to yeast pitch temp).
Buy a refractometer and check it at the boil kettle, during the sparge and on transfer to the fermenter.
OG = gravity prior to the start of fermentation FG = gravity after the completion of fermentation While the yeast won't start fermenting your wort for a while (often 6-18 hours) I still take it prior to pitching the yeast. I will typically pull a sample out of the boil kettle post boil / prior to chilling the full batch and use it to measure the OG
Yeast's impact on the gravity would be negligible, unless you're dumping in the spent starter wort too (i.e. not crashing and decanting). But I always take gravity readings before adding yeast. The reason is that by the time your yeast has been added to your painstakingly oxygenated wort, you'll want to get the fermenter buttoned up, to retain as much of that O2 as possible.
I'll add that taking the wort from the boil kettle while it's still hot has the advantage of being sanitary and easy (to access). Just remember to cool the wort sample down first before measuring its gravity and do a temperature correction adjustment when you take said gravity.
