Basic Question on alternate strumming.

Real beginner's question here. When doing alternate strumming, for example strumming up/down 8th notes in 4/4, how strict should you be about completing the last upstroke perfectly? I find it very difficult to make the change fast enough unless I use the final upstroke as a kind of transitional stroke, but this means that you aren't technically playing the chord correctly. Is it acceptable to play like this or should I always aim to complete the final upstroke before moving my hands off the chord and moving to the next one?