To keep the size of my collection manageable, I collect only chips that were playable as two and a half US Dollars. That usually means they have a dollar sign, but if it's known the casino allows play in the USD, I make an exception. I do exclude any that say 250, or 25, or have a currency sign other than $, particularly P, DR, and other south american currencies.

Mel and Lisa just sent me a 2,5 from Buenos Aires with no currency mark, which will go into the collection.