Bill - the 24 hour 'rule' is what the Customs and Border Protection agent at the Ely CBP station had told us. I have looked around on the sites for State Dept., US Customs and Border Protection, and the Western Hemisphere Travel Initiative pages and have not found a definitive answer to "when".  I suspect as soon as reasonably possible it perfectly acceptable - basically what you had been advised.  As for people never checking in, I'm sure there are many.  I would never recommend that (under the 'do whatever your career can stand' philosophy) and especially not with a Scout group - wrong example to set with young minds.  Plus, you just never know when things are going to catch up with you.  Looking forward to hear what you find out at your first aid course.

I took a course last year for our Scout trip to the Q in June through Wilderness First Aid (   (You need to Login or Register) and it was a really great course.  Their cert. is two years, cost was $240.  They teach only in the Mid-Atlantic states.  I'd recommend a wfa course to anybody - an ounce of prevetion. 

Thanks Solus, for pointing out the Enhanced Driver's License/Enhanced ID card is valid.  I overlooked that and it is all over the CBP site regarding the WHTI.  There are many more states and provinces issuing those now. 

I think the bottom line is to do your research and check the official US and Canadian sites.  It is so available on the web, there is no excuse.

pd