As a Canadian citizen residing in Canada I personally had no issues travelling back and forth to the US with the K1 status pending.
I did this during every step of the K1 process, including once it was approved but I was not yet ready to activate it. I was never asked to validate my intent to not immigrate but I did bring paper work back and forth with me to solidify my ties to Canada and need to return, just in case. I'm sure no two situations are alike but as stated before, it is at the custom's officers discretion.