Especially in serious/silent situations. If you are the source of the deed, is it better to ignore it, or just say sorry afterwards?
Short answer: Unless someone looks at you, ignore it. It always sounds a thousand times louder to you :)
However, if it is noticed, just smile, apologise with a reason (for example):
Oops, sorry! I haven't had lunch yet!
Sorry, I ate my lunch a bit quickly to make this meeting!
While grumbly stomaches or hiccups are pretty common in meetings (I'm terrible at this :) ), unless there is a medical reason (eg irritable bowel syndrome) there would normally be no time you'll be flatulating or belching in a meeting. If there is, simply apologise, state the medical reason and leave it at that :)
Note: If the unthinkable happens and you accidentally flatulate in a serious situation such as a meeting (or interview!), do not try to cover it with moving your chair! I saw a guy do this once in a meeting. You could see the poor guy trying to sink through the floor. I felt for him, he was so embarrassed. Nobody mentioned it to save him his dignity :)
Clear your throat, say "Excuse me.", then carry on with what you were doing.
If you're not presenting, then just keep your composure and be polite. If you are presenting, then if what you're presenting is worth listening to, then they'll keep listening to it.
You're only human. Just be discreet: anything more is asking for attention, and there's no need.
In some situations it might not be enough to say sorry.
If there's a window in the room, open it to allow for some fresh air to come in.
If you had accidentally dropped a mug, you wouldn't just say sorry either but rather try to clean up the mess as good as you can. It's a slipping hazard after all.
If that burrito death cloud slipped though your lines of defence, that's bad enough, but don't let it wreak havoc on your colleagues.
If you can feel it "coming":
I'd say it's better to let it out instead of waiting for it to find a different way out. Leave the room if possible, of course.
People become very uncomfortable when they cannot pee, poop, eat, etc. even though they really have to. This reduces the ability to concentrate on the meeting and thus compromises the quality of work.
I'd rather have somebody float above their chair than their mind being occupied with not doing it.
The polite thing to do is say, "Excuse me." and then carry on with the meeting.
However, it is more polite to avoid these things where possible. If this is something you feel coming on, suggest a rest room break. If the meeting is important, eat something before hand so that your stomach isn't growling. If this is more than a very rare event, you might want to seek medical treatment.