Well.. you shouldn't have to worry about it at all... ever... non issue. That's because a competent engineer and/or product manager should have spread the connectors out so there is plenty of power on a rail for any and all connectors attached to that rail.
Take, for example, this BFG:
You've got 22A, 22A, 36A, 36A.
For explaining this easily/quickly, I'll just focus on the +12V3 and +12V4. The rails are limited to 36A each. That's 432W. These two rails provide power to the four PCIe connectors; two each rail. One PCIe is 6-pin, delivering up to 75W per spec, and the other PCIe is 8-pin, delivering up to 150W per spec. That's a total of 225W. That means the rail can deliver almost twice what the connectors are actually designed to deliver. So... would the fact that this PSU has multiple +12V rails have any effect on anyone? Not at all.
So why bother with splitting up +12V rails?
Well.. have you ever plugged a connector in backwards or heard of anyone that has? The floppy power connector in particular is very easy to plug in backwards, whether it's plugged into a floppy drive, Audigy card... whatever, it happens and it happens often. Well, short circuit protection should shut down a PSU. If you touch a hot lead to a ground the PSU should shut down right away. But if you put some resistance in that circuit, like in the form of a sound card or floppy drive, the PSU doesn't see a short. It sees a big load. If this load is allowed to run without limit, the wire heats up, the insulation burns off and you end up with a heap of melted plastic at the bottom of the case. With a current limit, the load caused by the short exceeds the current limit and shuts down the PSU preventing the wire to ever get hot enough to start a fire. And that's ALL it does. So if someone tells you they're getting better OC's with a single +12V rail PSU, ask them for their eye color because they may be full of something. On the flip side, if someone tells you that voltages are more stable because they have multiple +12V rails, they're full of it too because typically the split rails come from the same source and the circuitry doing the splitting isn't doing any kind of regulation or anything that would actually make the voltages "more stable."
Hope this helps.