Depending on location, a fair bit of neutralization can occur because of the presence of ammonium. This question cannot be answered with strict percentages. It's importance is a function of the surface upon which it falls. Low acidity rain is generally anything above pH 5.0. High acidity rain is somewhere below about pH 4.5.
Medium acidity sloshes around somewhere in between.
The 1990 National Acid Precipitation Assessment Program State of the Science and Technology documents also provides comprehensive information in it's four volume set.