There's no "rule" but I'd say up to 5/10 online is where SSH is more applicable then HPFAP. SSH is strategy for playing against relatively bad players, while HPFAP is geared toward playing against decent players.

However, even if you don't plan to play higher than 5/10 for awhile, you should still read HPFAP. There are many concepts discussed that you should understand even if they may not apply to the games you usually play.