[BobJoeJim]

Well, if it was truly a "guaranteed victory" then yes, betting your entire bankroll would be advisable. Blindly betting big favorites with no specific reason for the bet other than "low risk, since they'll probably win" is a sure way to lose in the end. The lines on big favorites will have enough vig to be negative EV, just like the lines on underdogs will. I don't know for sure if the vig is better, worse, or the same on big favorites as on "normal" spreads, but I do know that it's there.

Keep in mind, that while your risk of losing any given game is low, so is the reward, and that one time you end up getting upset, you'll lose enough to cancel out a lot of small wins.

Now don't get me wrong, I'm not trying to convince you never to bet a favorite, but you need a tangible reason to think the bet is good. EV is all that matters on any bet, so you want to focus on finding bets that are +EV, rather than worrying about low risk.