I would remove it from MODEL.
I have removed it from MSDB in my environment with no ill effects but I am not doing any complicated development with DTS or SQLServerAgent jobs.
I am not sure about Distribution though. I treat replication like a handgrenade with a very loose pin. I think Steve Jones is the one to answer this.
It must depend on whether you are using push or pull subscriptions though I cannot be sure. I would suspect that pull subscriptions may need the guest account. The problem is that if you break replication it can be a pain to get it working again.
If you remove the guest account from a database you can always put it back by running sp_grantdbaccess 'guest' in your database.