Is it OK to Pay Invoices a little early or a little late?

I am working on a process improvement project in the Accounts Payable area of the company for which I work. They routinely pay invoices 1-4 days early or late so as not to have to change their regular pay cycle. Is this a widely accepted practice or is this just an excuse to be complacent with current performance? Does anyone know of any website where i can get information related to this issue? THANKS!