There's More Than One Way to Sacrifice a Creature!
What's worse than not having your own Commander? Maybe being beaten to death with it. The ol' "Stop hitting yourself" routine. If you only had a sacrifice outlet...
It's a terrible feeling when you spend your hard earned mana on a great big fattie, and someone else casts a stupid Control Magic on it. That's just one reason to run a sacrifice outlet, so we can end our creature's life humanely before our filthy opponent get's their hands on it! I think every deck should run at least a couple, but we can probably find ones that suit our deck's strategy better than simply plopping in Ashnod's Altar and Viscera Seer.
Those two sac outlets are the most common ones I come across, but I think we can do better. The altar will get you closer to casting your next bomby spell, while the seer at a single black mana is the most efficient of them all. This is probably why they're so prevalent, but let's see if we can't find some that are a little more interesting for what we're trying to accomplish!
White is fairly light in the sac outlet department, but has access to Fanatical Devotion, which can be a real boon in a deck that wants to go to the red zone. It's strange that white can't sacrifice well since it does a pretty good job at reanimating things. But when we pair it up with black, the king of sac, we get Vish Kal, Blood Arbiter. A flying 5/5 with lifelink that can freely sac any number of creatures to get enormous. The Orzhov pair also gives us Circle of Despair, which looks really valuable if your meta has a lot of voltron strategies or even infect. I'll point out that the effect lasts the entire turn, not just for the "next time" the source deals damage, as most of these effects do.
Not running black? Never fear! You've got options! If you're running green, I'd always recommend Perilous Forays. Why not get some extra ramp going for those creatures that are already marked for death anyway. How about green card draw? Greater Good is likely no surprise to you if you brew decks that utilize the graveyard. Drawing cards as well as filling your yard is a big play. No wonder that card is getting up there in price! Another angle green decks tend to take is to use a lot of +1/+1 counters, in which case, I'll remind you that Plaguemaw Beast is out there for the proliferate. Let's turn our opponent's removal spells into big gains!
As I've mentioned before, black is clearly the champion of sacrifice. I would be remiss to leave out some of it's sacrificial tools! Did you know black can search up lands? Me neither! But for the activated cost of 1B and a sac, Corpse Harvester can search up a swamp, as well as tutor for a zombie card! Alternately, (or in addition!) if you like a graveyard full of stuff, Undercity Informer does a great job as a sac outlet that can do just that. I find the informer to be largely overlooked, when it's such a useful ability! (In fact, as I write this, I'm going to add it to a couple of decks that could really use the self mill!)
So there are a few interesting sacrifice outlets for you to think about putting in your decks. There really are a lot of options beyond Ashnod's Altar. I wasn't going to say anything, but if you're super mean you could even run Sadistic Hypnotist, but I wouldn't recommend playing that against people you care about. It's sorcery speed only, but you're not playing this card for the insurance. You're straight up evil.