Very indefinite question. There are many more conditions to consider. If it was a one day game and defending anything less than 250, you would be a moron to be happy with 5/100, the rest of the match could be played safely at 3 or 4 runs an over. Many teams have lower order players (Hussey comes to mind) that could easily win from that position. Furthermore, you will find that expensive spell usually have friends, particularly if a captain is stretched enough to give such a bowler all his overs.
By much the same token, super-economical spells rarely come alone. If you find one bowler bowls 10 overs for 10 runs, you normally find his team mates backed him up. Even in a test, the pressure of a tight spell can spill over and turn up a gimme. Mind you, it's far more likely for a bowler to go wicketless for 10 or less off a full spell than it is for a bowler to take a bag and get carted, in fact even 5/80 has never occured in ODIs. Make no mistake, these performances do not win matches. Nazmul Hossain's 3/83 most certainly didn't put his team in any sort of position against England at Trent Bridge in last year's NatWest Series.