The honest answer to your question is that nobody knows for sure. Academic researchers (notably Adi Shamir, the S in RSA) have performed statistical analyses of the block chain and found various links, but nobody has been prosecuted by being linked to a crime through bitcoin transactions. It's almost certain that will happen eventually, but we don't know what LE capabilities are now or will be in the future, and how willing they will be to deploy those capabilities on criminals of varying stature. A terrorist or cartel boss has more reason to be worried than an SR vendor, who has more reason to be worried than a buyer. One thing to remember is that all transactions are saved in the block chain forever. 20 years from now, when LE has 10^20 greater computing power, those transactions will be available for analysis. I think it's worth paying a 3% fee for added security.