Mustache area is one of the few places I don't go against the grain.

I work straight down on the first pass, then I use the toe of the razor working in at a 45 degree angle to the nose but still working down but more to the opposite corner of my mouth on the second pass. If I do a third pass then it's across the grain, working from one corner of my mouth to center.

I have done against the grain passes on that area, And I have found that it always ends up with the shave been so close that it causes some skin damage, normally making you look like you used a dull razor.