Be wary of contractors who insist on cash.
A few contractors and service providers demand cash payments or offer substantial discounts for cash payments to avoid bounced checks, garnished wages, and Uncle Sam (although we have no direct proof of any company engaged in tax avoidance).
Additionally, some companies may in turn pay their employees in cash, thus allowing the employees to escape taxes and government regulations, such as green card requirements. Checks and other non-cash payment forms leave a paper trail, while hard cash has a way of disappearing.
If you ever need to prove you actually paid a bill, a cancelled check is very useful.