If your talking head rests, could be because of all of the gunk people put in their hair, I'm guilty as well, the head rests could need a steam more often then the regular seats. never really though of that, but it is probably true,
If it's the engine.. don't really see a point do only do one part, unless it is where you spend a majority of your time when you do clean the entire engine. Then it would make sence to "tidy it up" more often then the rest of the engine. You would lower the time of your entire engine steam, but only if the heads were the most time consuming part of the engine steam. Similar to how a weekly dust of your interior and shake out of the matts will reduce the time of your full interior detail.
Just my opinion..obviously
