Many service providers do not take a rate increase. They are just so happy to have the account the price stays the same till they are not happy working anymore, start doing a lousy job, get fired or quit.
I would say an increase is warranted each year as the cost of living goes up. If you bidded your account correctly from the start, you will be happy servicing that account. "A 4% CPI increase shall be added yearly to all of our contract work"
This can be pointed out to the customer prior to the signing, and explained just as that cost of living increase.
I agree, we generally renew all our contracts on a yearly basis. We point out to the customer that the increase is due in part to a CPI increase , wages etc.. We have customers that may tender the job out again but then, any commercial business you have has NUMEROUS companies coming in to give them quotes all the time. We have not lost a customer yet due to increase in price.