Personally, I would prefer at least 5 RB on my guards, RB in guards are pretty useful imo.
The reason I think many guards with low RB are able to get lots of rebounds in a particular game, is because there is a mismatch in RB skill, the defender( usually a guard as well ) has lower or same level of RB( since majority thinks that RB is a useless skill that jacks up salary on guards ).
Also, I tend to think that rebounding plays a very large part in winning games:
1. Offensive rebounds create more chances for your team to score( obviously )
2. Defensive rebounds avoid giving your opponent a second chance to score, avoid lost of 2 points while creating a chance for you to score, creating a 2 point, so its basically a 4 point play imho.
Anyhow, these are just my thoughts and opinion, might be wrong :)