Airlines do it all the time... Countless times, both Virgin & Qantas do the exactly the same...
I'd suggest that we (QF) do NOT do exactly the same. Personally I'm always ready to push when we request it. If something changes and for some reason I'm not able to push within 30 seconds of being granted approval I let ATC know. In my 13 years of domestic operations I've never seen it done any other way by any of the crew I've flown with.
Perhaps you'd like to re-phrase?