Microsoft's Major Nelson has issued a rather stern warning over Xbox Live account sharing. To put it simply: don't do it. Sharing your account can result permanently losing your Gamertag, Gamerscore, saved games and any points you have on the account.
In addition, Nelson warns against tampering with your Gamerscore and Achievements through the use of game save hacks. If discovered, you could possibly lose your Gamerscore. At worst, an account ban may even be issued.
Xbox LIVE account sharing and Gamesave tampering (don't do it) [Major Nelson]
