actually, what mattered more was the willingness to do the Roadblocks. all of the Final 3 females in Season 5 are so reluctant to do a single Roadblock, and for Christie and Nicole, as though they got Roadblock phobia after the caviar. they didn't even think of which member to perform the Roadblock regardless of whether it requires more skill than strength or whatever, they just want to play safe if it turns out to be a tough one. and it doesn't necessarily occur to co-ed couples - refer to Tian & Jaree. this is probably the trigger for starting the Roadblock rule.

and for Season 1-5 it wasn't even called a "Loose Roadblock" rule, there wasn't even a rule, but I suspect each team member must complete at least ONE roadblock before qualifying for final 3 or $1 million, since at one point of time earlier in the game they still have to decide who to complete the first few Roadblocks.