[member=5645]sillymoo[/member] is right; ideally you want a good match between the back odds and lay odds, which means odds as close as possible without being an arb (an arb is when the lay odds are lower than the back odds - not good for your account at all, do not do them). The reason for it is because the better the match/closer the odds are for a qualifying bet, the lower the qualifying loss will be and the better the match/closer the odds are for a free bet, the higher the return will be.
Examples:
1) if you have a £10 qualifying bet to do - at odds of 2.3/2.35 the qualifying loss is 33p. At odds of 2.3/2.7 the qualifying loss is £1.59. Massive difference.
2) if you have a £10 free bet to use - at odds of 3.5/3.55 the return is £6.94. At odds of 3.5 and 3.9 the return is £6.31. Again, a massive difference - especially when you consider how much it adds up if you're able to extract another 60p (or whatever) profit out of every £10 free bet.
Also, don't make a habit of going for 5.0+ for free bets as it's not good in the long-run for your account. You can get great value out of free bets with odds around 3.5-4.5, or alternatively using free bets on doubles/trebles using an acca sheet.