If you have mature grapes and want to impart lots of tannin it is generally acceptable to leave in 5-10% of the stems. If the grapes are a little on the unripe side (very green stems) or you want a more delicate wine I would remove all possible stems. A lot has to do with how tannic the wine is to start with... example I grow and make wine from Foch grapes. These have very little tannin in the grapes or seeds so 10% of the stems actually helps my wine.
I mixed 5 different mixes last year. Merlot, cab, petit Sarah, Grenache and Alicante. I didn’t open it yet but when I went to taste it it was a little vinegary. Just wanted to taste. So I waited until father day. Still a lily Le vinegary but not much. Now it’s been locked for a year and this has never happened before. All the wines came out great. Help I have 99.5 gallons of it.