proviron will have no negative effects on your gains at 100mg per day and below. it will have many positives. but remember that it will not help with any gyno problems that arise from the deca. for that you need to add some winny at 50mg eod or 25mg a day if you start to get the itchy nips.
IMO proviron should be a staple in all cycles. not only will it help stop aromitization, it will also keep the bloating down that leads to high blood pressure, not to mention at 50-100mg a day it will give a little boost to your cycle.