I have always either declined the upgrade or given the seat in front to my GF. On one occasion, offered an op-up CDG-BOS, I said I was traveling with her. They upgraded her too!

Funniest situation in this regard was at the end of 2005, the OAX-MEX leg of OAX-BOS, a short hop on Mexicana before two AA legs. My GF was on a J/F award. I was on a paid economy ticket - needed the miles to make EXP - and was able to upgrade the whole trip except this leg. She sat down in 4F or whatever it was. The seat next to her was open. I asked the FA if I could sit with her. He turned his back to the open front door, said in a loud voice so the gate agents would be sure to hear "I'm sorry, sir, you must go back to economy and sit in your assigned seat," while gesturing frantically in front of his body (which they couldn't see) for me to sit with her!

If tipping FAs was done, he would have gotten a big one.