Controling Special Access via Usergroups - Is my way the correct way?

[Quarterbore]

I have a few contributor groups on my website and the contributor groups gain additional permissions and forum access because of their contributions. I control this through adding "additional" usergroups to their account.

So, for example a normal member is is the "Registered Group". If a person donates $12 via a 1yr subscription they get added to a CONTRIBUTOR group (2nd group).

The CONTRIBUTOR group has the add-on group permissions such as:

Larger Avatar
More PMs
Access to Special (Members Only Forums)
Larger Photoserver limit

So, the primary group gives them access to the regular forums and this secondary CONTRIBUTOR group is what gives them the perks...

I am curious if this is the best way to do this? Should I be beassigning the Primary group to a CONTRIBUTOR group that gives them full access to all the REGISTERED + CONTRIBUTOR group has access to?

That could get really complicated for me as I have quite a few private forums that use individual controlled access so I really need to do it like I am with the Secondary group adding the special privlidges...

I just wanted to confirm as I was given a hint that I may not be doing this the best way but it is the only way I know to do this...

Thanks for any oppinions!