I"m nowhere near a professional, but I try to use the Straight Shot proc whenever I happen to get it simply because of that guaranteed crit that you get (which hits considerably harder than a non-crit Heavy Shot anyway, as mentioned before). I didn't figure it would result in a DPS loss (or if it did, it wouldn't be significant), and I can't think of a reason /not/ to use it unless there's straight-up nothing to target.