In the UK I've been taught to always select standby before changing the transponder codes. Not to do so seems very bad practice. Surely the SSR readout on the radar would be all over the place as you change the code? As well as the risk of passing through a 7xxx code, you stand the risk of passing through a code allocated to another aircraft. This may well upset various TCAS and Radar systems, whose software developers may not have considered that two returns with the same code is valid, so long as it only occurs momentarily.
UK 1 - US 0.