Interchange rates (the true cost to process credit cards from the card issuing banks) are usually updated semiannually, in April and October. Generally the changes are minimal increases. However, credit card processors use the opportunity to add fees and make increases trying to mask them as interchange. Processors have even gone as far as to add the word "Interchange" to their surcharge to try to make it seem like a pass-through cost instead of an additional cost they are using to increase profits.
Pay close attention to your April and October statements to see if there are any new fees that show up or increases to your processor's rates. Mastercard and Visa post their interchange rates on their websites in a downloadable pdf for merchants.