nitro just changes the mouthfeel of the beer. You don't necessarily need it ever (unless you have really long keg lines). I normally would use co2 for most beers with the exception of stouts, porters, and anything that is supposed to emulate a cask conditioned beer.
PrimaryLambic (WYeast blend), Local Lambic (spontaneous fermentation), Flanders Red with ECY02, Sour Rye (w/BugCounty)
SecondaryFlanders Red, Frambrozen, Grimm V2
Bottled Burton Ale, Wet Hopped IPA, Roese Kriek, "Grimm" RIS 23%, Saison Du Flood,"Warrior" IIPA 17%, "Oaked Warrior" IIPA 17%
Kegged Hip Hop IPA, Chocolate Cream Stout