I did my sub-i well after the interview season was finished and felt it was a colossal waste of my time, particularly since pathology residency does not require an intern year. My med school gave us the choice of medicine, peds, or surgery. I did IM simply because the hours were a little better.
You certainly don't need a sub-i done to be a competitive pathology applicant, and if your medical school doesn't require one at all, consider yourself lucky.
In terms of what rotations/electives to do during fourth year, you have many options. I did several pathology electives, including a CP month I set up on my own with a local esoteric testing facility and a research month, which were both fantastic. I personally tried to do as many pathology electives as possible, and as early in the year as I could schedule them (and perform well enough on them to get LORs). As other posters have mentioned, doing heme/onc, derm, GI, or a surgical service that generates lots of frozens and other specimens (i.e. transplant) is certainly an option. Or you could do something that you find interesting, even if it isn't path related, so you could have the experience before med school is over. Frankly, as long as your test scores are good and you have some decent LORs from pathologists, I got the impression that program directors don't really care all that much which elective clerkships you choose to do.
