Should I use the cash-out option offered by bookies and Betfair?

Many bookies offer a 'Cash-Out' feature, which allows you to lock in either a profit or a loss on an open bet. However, these cash-outs include a large overround which gives even more value to the bookie. This means that, mathematically, cashing-out at the bookie is never a good idea. For most bets you will be able to achieve the same thing, in a more profitable way, by placing a lay bet at Betfair instead. The only real advantage of using the cash-out feature is that it will make you look like a mug and may help with sustainability. 

Betfair also has a cash-out option, but it functions slightly different due to the fact that Betfair is an exchange and has no overround. When you cash out, Betfair will essentially place a 'back' bet to cancel out your 'lay' bet (or vice versa). Note that if the lay stake needed to cash out your bet is higher than the available liquidity, Betfair will automatically match it at the next available odds. These can sometimes be much worse than the current odds, so you need to be cautious. It is often better to place the bets manually yourself rather than relying on Betfair to do it automatically.