I absolutely agree that one should only gamble or bet with what one can afford to lose.  What I do not agree with is taking betting for fun. Many gamblers, myself inclusive, do not gamble just for the fun of it because we're hoping to make winnings and get something of it and it's because of that we do not just make guesses when we bet but calculated attempts at winning.
To answer your question, it's just a matter of strategy that works best for one. Personally, I prefer pre-match staking. I do this to avoid the tension that comes with watching a live match when one makes an in-play bet. What I do is simply make a calculated prediction, place a bet and wait for the result.