If I ever have to fart, I always let it go. If I don't, it comes to a point where I can't hold it back anymore, and I let it go anyways. I don't care what they're going to say; I let my farts rage on. Social norms never bothered me anyway.
If you're sitting beside someone and you had to fart, of course you have to hold it in because it would be unethical to release a fart while sitting beside someone. It would be deemed rude, as well. If you were alone and you had to fart, then you let the fart out since nobody will be there to hear you fart.
Specially when talking about girls. If you are a man you certainly have held in a fart for the girl not to think you are disgusting or anything similar. But I remember my father saying that he has heard about some cultures where it was polite to let farts out. Of course this is a cultural aspect, and it is hard to change, but maybe it should. Someone a couple of comments below said that it is not healthy to hold farts, so that would be another reason for not holding it. And this can turn into a wider and more polemical discution, about what is politeness. Because, if for some people it is polite to fart, and for some it is not, then politeness is (in some cases) relative (Protagoras!). And finally we shall come to ethics: Every single person should than, in my opinion, be allowed to let farts out, as it is a natural and healthy thing to do. People should remember, though, not to let this be an inconvenience to others (think on the classic elevator situation).
Holding in your farts can be very detrimental to your body! It can cause serious abdominal pain, buildup of gas, stomachaches, all sorts of nasty problems such as there. Avoid those health problems later in life. Be free with your body and spirit and soul, and let those farts loose.
It is trying to come out for a reason. So it's only logical to let it out. Too bad that it stinks but well...It's a good tool for revenge too. Also it's probably bad for you to keep holding in your farts. So be free and rip the fabric of space and time without your farts.