Chances are your tank is not obviously cycling due to the small fish load, as mentioned. You may not see a spike in ammonia but the bacteria will start growing, just slower according to the ammonia available... very low levels. This will produce nitrites in correspondingly low levels and nitrates will follow suit. Good for the fish but if you start adding more you may get the missing spike and, as Byron said, harm the fish.
I'd suggest going ahead and adding your plants as they will suck up any ammonia that is produced by your fish... enough plants and you avoid having to do the whole ammonia spike in the first place, better for the fish.
I have a 37 gallon with 13 fish, 40+ plants and have not seen any ammonia yet and the tank is about two weeks old.