When I was certified, I didn't tip my instructor. What I DID do was to let him talk to the women to whom I was married at the time while at a party. He was a real hound dog. I wasn't worried. Afterwards, my wife confirmed the fact that he thought he was God's gift to women. She said something to him to put him in his place that I'll be glad to share over a beer.
Back when I was going through all of my courses, tipping wasn't something that was on my radar. Since 2003, I have been tipping dive guides and dive masters with whom I have been diving. I have been offered tips, which I have declined saying that someday they can pay it forward.
I wouldn't feel guilty about not tipping. I don't think that instructors EXPECT tips, but as stated in the above posts, most will either accept graciously or have an alternative suggestion.
Cheers -
Edit: Punctuation and grammar.