It can be a hard call. Is that engine really leaking in a way liable to give you first-post rights on Rumours & News, or are you going to look like a nervous dolt? One time to Cork on one of MO'L's more venerable craft, I was sitting behind the right hand engine and couldn't help but notice large gobbets of oil occasionally emerging from an access panel and generously lubricating the back end. I'd watched lots of engines (going into Cork, it was those or the tops of clouds), and this was definitely something of a gusher.
Eventually convincing myself that doltage was no great sin, I had a quiet word with one of the cabin crew (which is hard to do without coming across as Mr Bonkers, but then pressing the call and yelling "Oy! Yer engine's about to ex-PLODE!" seems a bad idea).
Yes, I was being a nervous dolt. As someone said, it just proves there's oil in the thing. But I think that no matter what's bothering you, tell someone. It's what they're there for.
R