Paolo,
This is an interesting question. I teach aeros (usually in Bulldogs) and am not very consistent about 'chutes.
If I'm doing an aeros trial lesson I don't bother because the manouevres are usually simple - but sometimes the "stude" is unusually keen and by the end of 30 minutes we've been through the whole repertoire.
If I'm formally teaching aeros then it depends on what we're doing. Always worn for spinning, usually worn for ROTs, S/Ts, recoveries from deliberately botched manouevres and flicks. At other times it's down to the stude to choose, we make the 'chutes available but some people don't like wearing them and others do.
It often occurs to me that it would be pretty interesting to be in a seriously sad aeroplane wishing I'd spent the extra minute to put the 'chute on!
One further thought. If I'm wearing a 'chute I (and the stude) always wear bone domes. The rationale for this is simply that the only things that would make me step out would be serious flames or a major bit breaking off. In the latter case the hairyplane will be doing something unusual and there's probably a fair chance of being wacked about the bonce!
BTW. If you look at the US aero fatalities you'll see that one was in a Seneca!