This year was far worse than normal (thanks to Oden and Durant), but yes, it does usually happen. The Spurs did it to get Duncan, the Cavaliers did it and got LeBron James, for example.
The last two years the 49ers actually won games late in the season (they had a shot at Reggie Bush two years ago) even though everybody knew they sucked. Nobody would have complained if they had kept losing. But it turned out just fine for them. I think since football has so many more players, and #1 QBs can be a crapshoot anyway, there is less incentive to tank. I would only tank for a franchise QB or RB, but in basketball you can get a franchise player at any position.