The funny thing about dancing specific length phrases is that the only people who will know if you pull it off are you and your partner... no judge is going to watch you for 8 bars unless you are alone or nearly so.

However, a judge might as you suggest notice if the pattern of stress in your dancing matches that of the music at a given point in time, or at least if it seriously mis-matches it. So that's what you need to worry about - not getting exactly 8 bars, but simply avoiding conflict with the music.

Every one of the standard dances has a two-piece pattern of musical units that are technically interchangeable, but still different enough that dance figures fit to them one way a bit better than they do the other. For tango, foxtrot, and quickstep, this unit is the slow or half-measure, while for both waltzes it is the whole measure. Unfortunatley, a lot of groupings take an odd number of these units, and insertions you might like to make for floorcraft reasons can throw things off as well.

And there are also longer phrases as you mentioned, which come with their own highlights in the music. Those would be points where you would especially want to have your figures in the ideal relationship to the two-piece pattern, and if you could even go so far as to pick something somewhat climatic.

Figuring out how to do it all is of course hard. One thing to do is to preplan phrased routines or at least walls. Another is to listen to the music as you dance, try to hear where it is going (either in terms of the two-piece pattern or the full phrase) and then make plans to catch the music, either immediately, or perhaps intentionally delaying this resolution a bit into the future to build up energy.