maybe not tanking per se. Maybe not going all out. Tanking to me implies purposely losing. I think, the strategy would be to get the feel of the surface, but don't hurt yourself, don't go crazy. If you lose you lose but try and win. I also think this works for those seeds whose ranking is high.
See Tsonga for a good reason why one would be careful with a slam 2 weeks away.
Garion summarized it very well imo, especially in Rafa's case in grass tournaments before Wimbledon.
That being said, all-out tanking also takes place. Just like any professional going to his workday has days where he tanks, it happens in tennis also for various reasons. For example, Kafelnikov tanked completely against Hugo Armando (a player who never cracked top 100 and remained a challenger-satellite level player most of his 12-year career, his highest ranking was 100, and that came after that week) in the 2nd round of Kitzbuhel in 2001, when he was seeded 2nd. The score was 6/1 6/1 in 45 minutes, and Kafelnikov did not even hide the fact that he was tanking. I remember him double-faulting, going for a huge second serve, and when he was whistled by the crowd, he pushed the first serve in, then zapped the next shot straight to the wall on the other side of the court, as if to mock the crowd (it also possible that the match was fixed).
Also Federer pretty much tanked against Volandri in Rome in 2007, 6/2 6/4. He did not really make it look respectable by not going all-out like Rafa did yesterday, his body language showed clearly that he did not want to be there, Volandri said as much after the match also, but also punctuated that he was not complaining.
Sometimes players need to leave urgently for a reason or another, or simply regret singing up for the tournament becuase circumstances are different now then when they signed up, but instead of not showing up and possibly getting fined, they will show up and tank. I believe (although I am not as sure), this is what happened to Tsonga in Cincinnati in 2009, when he played a great Canadian Open the week before, losing in the semis to Murray, then came and lost Chris Guccione in Cincinnati first round. He actually tried in the first set (in a respectable way, not all-out), and lost it in a very close tiebreaker, and then he switched the lights off, losing the second set 6/2 in about 20 minutes I believe. He was rushing between points at warp speed and looked completely ready to leave after the switch over at the end of the first set. He won the two games because he served an ace and nailed shots that happened to go in, or Guccione simply made mistakes.
Anyhow, tanking does happen, but again, rarely. Most are the cases such as garion pointed out.