I agree with the wait-six-months-between-submissions approach. This shows you've been working on your craft and striving to imiprove -- that you aren't just throwing out a ton of manuscripts to see what sticks. But, Kelly has a point about timing -- if you sub and three months later get a rejection, you might only have to wait a month or two before you send the next story. Honestly, the PB market for unagented subs is so small that either they are inundated with subs, in which case yours might get forgotten in the chaos and a new sub won't even register, or in the small arena you might stand out (for better or worse), and a new, sudden sub might be off-putting.
Gah. So, yeah. Guess that wasn't really helpful!!