If you send bleeding units into the co-op queue and order them to be healed in the barracks before your match starts, they will heal during the match and you won't have to pay to heal them again after the match. This can give some unfair advantage for those that figured this out... (unless this was intended, but since you can't bring healing units into matches... I doubt it)
Haven't tested it, but it probably happens with sending units into PvP as well.