I think this comes from urban legend, that some manufacturers don't explicitly mention the idea of topping off in their instructions, and just list magazine capacity in their specifications. Some infer from this that the manufacturer doesn't approve of topping off. Or this could come from the old school mentality of making the case that semi-autos aren't reliable, so that school of thought makes up the excuse that there's going to be so much friction from the top round in the magazine it will slow down the slide and cause a jam.

As long as your pistol can physically accept a fully loaded magazine with the slide forward without resorting to bashing the magazine in and deforming the top round, I don't think there is going to be any problem with +1.