I've been pondering this myself for the past few nights for my Subversion repository setup. What about this. I helped to start a small computer shop which led me to the coding, so this is based more on experience than logic. You all should already have a unique ID for your clients for records anyway, so leverage that. Dedicate 000000 or equivalent to in-house software or official releases for sale. All other numbers will be for a custom job from the corresponding client, like websites. I should clarify... below is the directory structure. Hope this helps. :)
000000 - (Client# always belongs to: Inhouse)
(Trunk, Branch, Tags, etc.)
(Trunk, Branch, Tags, etc.)
000001 - (Client# belongs to: John Doe in NC according to records.)
(Trunk, Branch, Tags, etc.)
(Trunk, Branch, Tags, etc.)
{...}
002731 - (Client# belongs to: John Doe in LA according to records.)
(Trunk, Branch, Tags, etc.)
(Trunk, Branch, Tags, etc.)
