MathProf just pointed out in another thread
That the value of fives has mainly to do with your edge and not your playing strategy. Cards that you track seperately are mainly tracked so that you can alter your play strategy. The aces, for example, are valuable but have little significance in play strategy thus subtracting their high card value for playing purposes can be helpful, also since they sometimes behave like a small card and sometimes as a big one. Sevens also sometimes have that property and play strategy can be improved by tracking them. In the better count systems the five is already highly valued in the primary count and will show in your advantage, and thus your betting, when they are removed. But there is little value in tracking them for purposes of play strategy.