It is possible that some seals or diaphrams in your carburettor may not be compatible with alcohol containing fuels. The plastic tank is not likely to be affected unless it has some inherent moulding faults, where the alcohol could promote stress cracking. Some people have reported problems with plastic tanks, which they think gasohol has caused. Other newer bikes such as the BMW's often have plastic tanks which are fully gasohol compatible.
Maybe safer for you to stay away from gasohol unless you can readily get replacement rubber parts for the carburettor.