The guard flag is only given to servers that are sufficiently fast and stable [[dir-spec][dir-spec:guard]].  Stable means being available most of the time.

So yes, if your relay goes down due to hibernation this can result in your relay not getting the next time it becomes available.  Clients will deal with this and just pick new/more guards.

I am not aware of any means to refuse the guard flag.

 [dir-spec:guard]: https://gitweb.torproject.org/torspec.git/blob/ecf9d02b5bf3d17fd90630f006591768a7473c22:/dir-spec.txt#l1775