This question doesn't seem very easy to answer.
There are plenty of guys who are not on T and did have the surgery. As far as I know, they don't have any issues. At least, not on the short term. Perhaps issues would develop on the long term? But then there are cis guys who develop breast tissue due to an excess of estrogene (which a transguy not on T also would have). So how this works, I am not sure. If you wish to know, ask a doctor or endocrinologist.
Also, what I've read is that some surgeons prefer people to have been on T for a little while as pecs develop some, which helps them sculpt the chest more. Again, this could just be rumours and the person to ask would be your own surgeon as even preferences between surgeons could be different.