I can't speak for everybody, but there's definitely no reluctance to boo. The team have been booed at half time and full time for months. The thing that people need to realise with our match going fans is that we pride ourselves on the way we support the team. The money men get less respect (see Green & Gold campaign). But it would take something particularly bad for the fans let all their feelings be know at the games.
There's always been this arrogance, in a way, around our match day support in the way that we laugh at the way other clubs turn on their managers and players. Nobody wants us to end up like that. I can't say what the breaking point would be, but I'd guess that a loss to Liverpool will be the starting point. If we ended up in a situation where we couldn't make top 4 then I reckon the fans would turn. Even in saying that though, I find it hard to imagine. We're just too respectful of our footballing figures to give them too much abuse.