There are basically two types of truss used, not being there can't say which is used.
Many people have mention attic stairs so I assume many R storing stuff up there.
It must be noted that is you put any decking up there you should NOT crush the insulation, in fact there should ideally be air space between same.
You can use plywood or strand board, but to save weight and make it easy to get thru your attic stair opening if you go to HD have them cut the pieces into 3 equal parts about 16" x 96" and space them apart for air circulation and better coverage.
The garage has a span of about 20' so be mindful of that if anything is heavy, it is better to place that stuff over interior walls if you can for better support.
The first type light storage is design for 20 pounds per square foot (PSF) to the bottom chord.
http://www.wood-truss-design.com/ima...e-600Wx24H.png
The other type is a true attic truss for heavier loads upto about 35# per sq',
you can clearly see the bottom chord is bigger like a 2x6 or 2x8.
So if you have this type it can support more weight than the first type.
http://www.wood-truss-design.com/ima...ain600x236.png
Bottom line don't overload spread out the boxes in my unprofessional "guess" you should be ok.
