Just asked ATC
AOPA (US) just issued a short movie about this (Dec 30th for you members). It it password protected so I cannot link to it, but the message is: ATC asks pilots NOT to switch the transponder mode to standby unless specifically asked to do so. ATC expects to lose the numeric code during code cycling, but do not want to lose track and altitude. No mention is made of old or new transponder types.
At an instructor's meeting in my flying club (in Denmark) this came up last year since we had strong and different opinions about it.
I brought the question to the chief of ATC controllers at our airport, and he initially said that he could not care less, then added that he was leaning towards always keep the active mode. I then presented him to a selection of the horror stories I - and some of you - have heard about what could happen if we struck some of the tricky squawk codes during cycling, without selecting standby. He just shaked his head, never having experienced or heard of ATC problems in that respect. So now we teach our PPL students to always leave the transponder in active mode.
Personally I still catch myself selecting standby when cycling old transponders with dial type code selectors. Old habits ...