Do I tell my children the truth about Santa Clause?

I feel there's no harm in letting your kids discover the truth on their own. Eventually they'll figure it out. I certainly wouldn't lie to them if they asked but rather simply tell the truth.

if at age 18 your children still talk about waiting for Santa to bring presents, perhaps then, it would be wise to let them down easy. 

I should think they ought to be nice to their parents even if they don't get gifts for this...

I wouldn't tell them before they're 8 or 9. As tiptomanger mentioned, most kids find out for themselves anyway, and then you can just lend moral support and explain in a way they can understand.