Many of us were taught as children that if you were offered refreshments when visiting another's home, the polite thing to do was to refuse "a first time", since hosts would probably insist a second or third time.
The trouble with this is that, if a host is really up on his/her Peas and Q's, he/she will not insist you accept food/drink after turning it down so as not to put a guest on the spot and embarrass them. So if you really would like to partake of the refreshments offered, accept graciously without false politeness the first time the offer is made.