I am equally going to stay away from the innuendo (heh heh, he said in you end o)
I do believe that some grains will be more prone to leach tannins. That is based on the comments of a craft brewer who uses a lot of decoctions when he was asked about tannins and that process.
Regardless, I don't think that compressing grains will leach tannins.
If you look at industrial scale mash filters (click the process animation on this page) you see that the grains get compacted into massive cakes. Since these are primarily used in BMC type breweries, you can be sure there is no risk of tannins getting leached into those delicate (weak) flavor profiles.
I asked the brew wizard about this and didn't hear back (although I didn't get the Dec. issue, a separate thread on that if BYO customer service doesn't fix that for me). Then I had the chance to talk to Charlie Papazian and asked him about it. We both agreed it must be a myth and got back to drinking beer.
If I had to guess, I would say that the risk would come from over sparging if you were to squeeze most of the liquid out, then add a lot of of sparge water and squeeze it again.