Hey,

I agree with the above answers that genetic drift, the major concern with using cells of high number of passages is highly dependent on the cell type and cancer cells do have a tendency to mutate quite frequently. Of course once you start passing cells too long, things like mycoplasma, etc. also become an issue and it's best to start with a fresh stock.  But, for preliminary studies, that are backed up with tumor studies, it is acceptable to use cells with high passage number.

One exception however, if you culture cells directly obtained from primary tumor, you probably don't want to go beyond 2 or 3 passages at most.