The rules don’t say anything about trimming the excess bag material. Any one know if we’ll get in trouble for doing so?
Why do that? Just don’t and then you don’t have to worry about it.
i like things neat. All that excess is messy.
I mean, you could submit that (and I suggest you do) to the FRC Q&A, but I suspect they might just tell you to deal with it.
inspectors don’t measure the bag. I don’t think there should be a problem with trimming it above the seal. see the recent Q&A on missing bags for other discussion directly from the GDC.
Don’t forget to bag your other spare parts and such as well, which may or may not take up more space in the bag.
I know teams that have used a bag multiple years so I’d be wary of cutting your bag.
The excess bag can act as cushion while transporting robot. 
Each season you get two bags, so you don’t have to worry about next year. Next year robot dimension could be smaller than this…just to challenge teams!