Nothing serious, you get a little when sitting without a strap. Its fine on even the cheapest strap.
I went to a guitar shop a few weeks ago and tried a few different basses. The balance on the knee is comparable to a Fender player precision that I tried.
Incidentally, I tried a sandberg electra vs4 and although sounding immense, the neck dive on my knee was too much and I could hardly play. I hope this was a one off - beautiful bass otherwise.