Whether or not they count it probably won't matter if you're buying right, after awhile. Your DTI will be so good from the ones they are counting, that an extra one won't budge the DTI much.
It's the factor that AR listed: 1-4 are easy to get, 5-10 more hassle, and then the traditional banks mostly cut you off, and you have to get more creative.
It's something to start thinking about when you've got a half-dozen plus properties. Before then (and especially below 4), don't worry about it. Take the cheap institutional, low-rate, fixed mortgages while you can, IMO.