Absolutely get a visa.

As a British citizen, Canadian resident, I have been refused entry or extremely delayed. It became a bit easier after I became Canadian but only because they decided to give my Canadian born wife a difficult time.

I never make an absolute plan of crossing the border.

I'll try to avoid the political so read the following as ironic. I personally know Brits who have served with Americans: denied entry. Brits with children in combat right now, RAF and SAS: denied entry. Brit seniors in wheelchairs: denied entry.

The important thing is this: what is posted as a guide to visitors means nothing to the Border official. That person can arbitrarily make an exception and there is nothing you, or your gov't, can do about it at that moment.

Get a visa, long before you hit the US border.

BTW: My entry to the US last year needed a temporary visa. There were 6 questions to answer, two were: did I belong to a terrorist organization...; and did I intend to kidnap American women. If I signed yes to any of these, I would have to fill out another form. Go figure.