Also, the individual user doesn't know that someone else has an SPFYou misunderstand. I mean for that option to apply to messages TO that subscriber, not FROM them.
My thought is that users whose email service has this characteristic might want to be able to join any group - and not be reliant on convincing owners/mods to opt to munge header-Froms.
So doing -both-, in my opinion, would be optimal but a minimum shouldI think it much more likely that the subscriber using the affected service would know about the problem. Group mods/owners have enough to cope with without making them responsible for knowing the ins and outs of every email service - or even for knowing what services their members use.
But in any case, if it seems desirable to give the group some input to the option (groups on the topic of Exchange or Office-365 usage may represent use cases for this), then I would suggest doing it like a subscriber's Time Zone: the group's Default Sub Settings tab can provide a default that applies to new accounts that join the group. This way users with established accounts would not suddenly have their preference changed merely by joining a group where that is set.