Well, it depends if they need it or not. Eg. If it's not going to improve their quality of life one iota, with the only effect being an increased IHT bill when they die, it'd be stupid to pay back house deposit money etc.
In my situation, my parents worked their rears off to put us in good schools, and move us from the countries we lived in (which costs money) and give us a good quality of life (such as holidays and good food etc...).
I had to learn this for myself, and they helped me out (this was well over a decade ago). They couldn't help me out for a deposit (but I never even expected them to help me out of my financial situation - I didn't need their help, it just accelerated its resolution), that took many many years of saving - but they helped bring me back into a more stable financial position. Both of them are still working and it was only fair for me to pay back my debt to them.
I'd like to think that had they been multi millionaires I'd have still paid them back, as it is a matter of principle. I don't like the taking for granted that people seem to have for their parents. It seems a little unfair to take and not to return?
Should your parents offer you money for a deposit, you could always pay them back in monthly installments so it would be like a boost to their pension. I'd rather they not have to worry about money when they're not working.