My future sister-in-law is the same way with our wedding. She does the offering again and again after being told "No, thanks" thing. Each time with a self-deprecating "I know I already asked, but I just wanted to offer" or "I know you're probably sick of me asking". Finally I jokingly just said "

eah, knock it off, will you?" and it has still happened once since then. MIL is a bit of a different situation though.
Sounds like the best thing is just what you're doing. Just remember that she's getting old and probably feeling old too since retiring. Sometimes people just want someone to appreciate their contribution to something and to feel useful, and they just don't think about whether you need it or not.
I take it on a case-by-case, much like people that are fishing for compliments. Sometimes they are extremely annoying and shouldn't be encouraged, but other times tossing them a compliment is not hard to do and they could probably use one.
-----signature-----
College women are as unstable as free radicals. Thanks college.
-Anebriated
He was a jedi before he was a father and everyone knows how much jedi love to chop off hands.
-Kigaro