I busted my ass on my route as a sub/regular for about 15 years, if, person wanted to give me cash/gift card during the holidays, I took it, regardless of the amount. I shared the cash with my sub (rarely had one), & one or two clerks who helped us get out of the building.
I believe the reason ppl are not supposed to accept cash/gc is all customers are supposed to be treated equally. If someone gave you $100 at the holidays & someone gave you $0, who would you maybe do something extra for. I know, I did.