Pending how old the PSU is, and ATX standard more than likely to be a single rail PSU, unless it is a super old PSU. Multi rail PSU's aren't a thing anymore, and have been in declined for a couple of years now, due to the shift and focus to 12volt generation, and needing less and less 3.3 and 5 volt generation, this resulted in new topologies that take advantage of it.
You most likely have a single rail PSU, and technically speaking it wouldn't matter which cable or how many cables you draw from as it all comes from the same rail, so it really doesn't matter.
If you have a multi rail PSU it is an old topology and wouldn't trust it as far as I can throw it with a modern system.
The question rather should be how good the OCP is and whether it is overzealous with spike loads
Posting your exact model number of the PSU will help to figure out what is what.... But yeah, load balancing on the 12volt rail hasn't been a thing for years, due newer topologies and single rails