Notes don't have to be absolutely pristine but will be rejected if they are marked or damaged.
The main things that are checked by Bulgarian bank cashiers are marks or writing (
e.g. where a UK cashier has written the total of a stack of notes on the top one) and damage, e.g. tears. missing corners, etc.
If you have (or open) a Nationwide Flex Account and use the associated Visa Debit Card to withdraw Leva from Bulgarian ATMs, you will receive a better exchange rate than you will for cash, with no charges - this is the only method that I tend to use these days. It's even better than using one of my Bulgarian
Borica bank cards, which charge 20 Stotinki per withdrawal.
Baldur