Having all slots populated does make extreme OCing more difficult, yes.
But this was more specifically relevant in the days of s775, especially toward the end with 45nm quad cores, as you generally needed a very high FSB to reach higher OCs, which put a lot of stress on the NB.
If you were running a lower FSB on a good board, i never really found all slots populated being a big issue.
Good board/perfect voltages/settings were important though; 4 DIMMs heavy OCing on s775 isn't usually n00b-friendly.
For P55/X58 with less of a need for extremely high BCLKs in many cases, it seems you will usually run into a CPU cooling limitation before anything else, unless you are doing good water-or-better cooling.
Also, there are always lower RAM ratios available, so it's again less likely for RAM to be holding things back.
In short, for the time being anyway, i would say populating all slots in H55/P55/X58 is less of an issue with regards to OCing.