Money Etiquette

So, I think I know Brian's opinion on the matter, or rather maybe I am biased by his experience, but I have a question for blog-land.  I was talking with a guy I met last week, and he asked me for backup on a disagreement he'd had with his spouse.  Being a good member of the species I naturally agreed, but it so happens that I actually DO agree with him on this one.

The question is, what is the proper use of personal cheque?  More specifically, in person-to-person interactions, is there a minimum amount below which it is really more polite to use cash?  (Brian, this is an invite to use a name-free account of a certain Ramunto's trip.)  When forced to give money to an institution, naturally we use cheque since they demand the paper trail, and we don't care if Bank of America has to go cash a cheque for $14.97 instead of us giving them a Lincoln and Hamilton, keep the change.

This actually turned out to be a relevant question for me as well, given the Craigslisting that we've done this past week.  We've paid with cash,  but how much does the $$ total have to be before it's acceptable to write a cheque?  I'd say at least $60 or so, and more generally a cheque should not be written for an amount that a reasonable person (not an undergrad or grad student) could be expected to have in the their wallet.