Is it safe to add shelving to truss members in my attic?
Trusses are engineered to support loads that are applied in certain ways.  It would often not be practical to make a truss support all of its required loads without it also being able to support some additional load applied in other places, but it's generally unwise to expect trusses to support much additional loading beyond their own weight.  For example, a truss which intended only to hold up a roof above it (and not a ceiling below) might well use a thinner bottom board than would be required to support a ceiling, and one which is intended only to support a ceiling might use a thinner board than one which is designed to support both a ceiling and a storage area.

Adding loads to a truss which are small relative to the weight of the truss itself (e.g. an attic light fixture) should be safe, but adding loads beyond that could be dangerous.  Note that applying a load in the middle of a truss member will create stress on that member proportional to the ratio of its length to its thickness.  If a truss has an 8' long 2x4 board with nothing attached to the middle of it, each pound of loading to the middle may increase the stress on some parts of the board by more than twenty pounds.  It's not hard to design trusses to withstand interior loading, but such trusses would cost more than those that aren't designed for such use.