Join Date: Sep 2005
Location: HEL
Programs: AY Platinum, TK Elite, BT Executive, AA, BA, SK, DL, NT, WB + hotels
Posts: 10,256
This depends on which airport you are leaving from. They used to weigh my hand luggage routinely at LHR back in the days (before zone R in T1) but as of now, I would say they are not strict at all.
AY has their own staff at JFK and my experiences there are quite positive. I would say that if your bag looks at least remotely cabin-size they will probably automatically assume you will take it as carry-on.