How about a standard this year, everyone puts a sticker on their robot listing the weight of the robot. Therefore, when you are on an alliance you know what robot is the heaviest and should be used for balancing.
119.9, all the time. 
Or you can just talk to your alliance partners before the match, as your total weight doesn’t include bumpers, and you might have 20 pound bumpers, while your alliance partner only has 16 pound bumpers.
As long as the teams know, they can talk to each other before the match. I don’t think a sticker would be necessary, but it is definitely something that should be remembered by the drive team.
Both very sensible and practical suggestions.
I will be difficult to balance even if all of the robots are the same weight; weight distribution will affect the ease of balancing just as much as the actual weight of the robot. In the end, the best that you can do is talk to your alliance partners.
Of course, that’s not entirely true. We could all build the same robot so that we know all of the dimensions of each others’ robots before we show up for competition, but what would be the fun in that?