I normally cut them off come winter but I have done it other times of year also I can’t seem to truly tell any difference on time of year and pruning them off. Some root stock just seems to sucker more than others. I have a semi standard Macintosh and two Red Delicious that are bad about the root suckers they where all purchased from the same source many years ago and I am uncertain of the root stock. I have Harrelson, Enterprise, Liberty, Fireside, William’s Pride and Freedom all planted in the same orchard that never root sucker but I do believe they are on a different route stock at least they where ordered from a different vendor. This was back before I was particular about the root stock I ordered. My current favorite for my soil is M111.