I don't think this is necessarily the case anymore. Looking at the major casinos on the Vegas Strip and Downtown from my BOLO collection, there are about equal numbers of white base and blue base $1s. In the rest of Nevada, the ratio doesn't favour white by a large margin, I'd estimate maybe 60:40 in favour of white from my non-Vegas BOLO samples.
Perhaps non-Nevada, non-California casinos still prefer white base for $1s.