I'd say you're right. When I started around 10 years ago at the infamous VIP down on Broadway/Canal in C-town, I had no idea that we needed to tip these girls. No one ever asked me for one. Not once. Since then, I've moved on to K-girls in Midtown and occasionally out in Queens - and until around 3 years ago, it never even dawned on me that I'd have to tip them. But once I started, it feels like now the girls always seem to expect one. When we're done, there's often an annoying moment when either I'm asked for a tip point-blank, or an awkward moment when it's implied.
Either way, I started off low and I've tried to keep that way. But I agree with all the random posts in this thread and many others, these K-girls definitely know how to negotiate for more. I've also tried the "All I have left is $40" or "$60" routine and usually it works. In the end, it's odd to me that in the past I was never even hit up for a tip before. Maybe I just look more like a sucker now.