Tenant paying rent with cash - issue or not?

[Virginia Jones]

For about two years now, one of our tenants has been paying for rent in cash. I am almost certain nothing illegal is being done to obtain this cash - they are in a band and receive tips for that and his wife is a waitress and gets tips. He works at the same place as my husband so no extra effort is being made to go to the house and retrieve the rent. 

I've always heard not to accept cash but we are having no issues over it. Is there something I'm missing and at what point would I need to insist they use a different method?