Well, you're in for a lifetime of blood tests, period. We all are. No one is assumed to be cured - we're all on surveillance. This is for a number of reasons - late relapses occur, the odds of a second primary are higher in those who have had TC, etc.
Now, with respect to the bHCG levels, I would ask TC Destroyer. He once showed a plot of his bHCG levels versus time - I can't find it right now. I am not sure how his have been in the long-term steady state, but I have heard of others whose levels remain persistently on the higher end of normal, or even a little above.
Also keep in mind that "normal" can also be a function of the lab and the equipment being used. The first time they ran my levels, it was at the Mayo Clinic's lab and they claimed on that test that anything above 0.7 was abnormal. My oncologist who has a research background said he would honestly be surprised if they really could ascertain the value with such precision in a high-throughput setting. I've heard of "normal" being anywhere from less than 1 to less than 8 or so, depending on the test and the level of precision/accuracy that they wish to attribute to their methods.