Is it okay to use staples instead of nails for shingles?
Galvanized steel or other corrosion-resistant nails are highly recommended for installing shingles.
Your contractor should only use staples if that is what the shingle manufacturer recommends. Other than that, nails should always be used. This makes sure that they are held down properly and won't move for years to come. When staples are used, it is much easier for wind to get underneath the shingles and rip them off.