Acceptable to simply IDENT when requested without verbal acknowledgement, or not?
Not acceptable in the UK - see CAP413 Chapter 5 Radar Phraseology :
5.9
The pilot must respond to SSR instructions,
reading back specific settings.
"BIGJET 347, squawk ident "
"Squawk ident, BIGJET 347"
As fredix says above, if the wrong aircraft squawks, all sorts of issues can arise.