Yes, I guess so. Can't do any harm, and there's always a chance that it may come back. I agree with Walker - it's more likely to be the dampers than the print head. Unlike the print head, these are not that expensive to replace and people who are handy can do that. But if you're content with a glossy P2 printer, then there's no need and you can always watch what happens to M over time.
If you go this route, don't be surprised if the printer does a silent switch to MK and back once every three to six months, to exercise the selector switch. There was a (highly debated) claim on the luminous landscape forum recently that the 3880 printer does this. I've never seen it, and this was the only report I've seen, and if its true it might not apply to the 3800, but it's another good reason to have flush in the MK cart and lines.
A final observation for the same of completeness. If you were to print on matte only, then I'd swap MK to Y and put flush in both the K carts. There are quite a few 3800/3880 out there that have had a faulty black selector switch, either due to sedimentation or a plain old faulty part, and these cause a world of pain. Black carts mysteriously drain, or one or both black channels won't print etc. On an older printer, the repair cost is probably not economic. (People who are really handy can replace these themselves as well - the spare part is not the expensive part, it's the labour.) You only need six channels for matte P2, and so in this case I would put flush in both black channels, as it's the best chance of keeping the printer going. But this is not an option for gloss - you need all seven working channels, six for ink and one for GO. For matte-only K6, the 1430 is a cheaper option.