constant dilemma!
we have only a joint account, and my wife is fastidious in reconciling receipts and bank accounts in Excel - she's a bank manager's daughter!
not really a fine toothcomb, but questions do get asked, so i have to be prepared.
Generally, it's as others have done - tenner here, 20 there, and stockpiled somewhere well hidden. takes a while to build up a sizeable wad.