I finally replaced the air filters / cleaned the coils for the first time, after 100k miles, 5 1/2 yrs. Never had a real problem with the odor although I would notice a very slight smell from time to time during wet weather, when I'd do multiple back to back trips (high humidity situations). This was not unlike any other car I've ever owned, so it never appeared to be an issue. I'd purchsed the filters (including one for the top opening under the hood), and some "Kool It" cleaning solution maybe 3 years ago, and just never got around to it.
What surprised me was the condition of the filters after all that time. They were dirty, yes, but only slightly so. I noted a very small amount of what looked like tiny grass clippings in the folds of the filter closest to the intake, but that was about it. I live in Florida, and even with our warm weather (and frequent rain showers) I'd think I'd see more of this problem, but in all this time it has been very tame. Possibly it has to do with the fact the car's garage kept, like, 95% of the time its parked?