Part of the problem with customs is that you just never know. All it takes is the wrong agent at the wrong time and the nightmare begins. Having said that, I think land crossings are probably more risky than air crossings. At airports the main concern seems to be safety and there is generally a much higher volume of traffic. Land crossings are less about safety and more about "are you bringing in any goods that we can nail you on". The volume of travellers tends to be much more diverse as well. Slow day the border crossing? Let's bring this guy inside and see if we can find something on him. In my experience it's true that most agents don't dig very deep where a bass' wood is concerned but, again, all it takes is one agent who decides he wants to prove something (and who knows whether or not the stricter regulations will mean changes in agent behaviour). I already find border crossings stressful enough when I know everything is legit!
A couple of years ago I was bringing a bass across the border from New York to Quebec by land. It had a rosewood fretboard. I had the receipts for the bass including documentation of the wood specs. I ended up being detained for around 35 minutes as the customs agent surfed the internet looking for price listings for similar bass models. One of her comments was "I know they use rosewood on some very expensive guitars". Yep, they also use it on some very cheap guitars. Her concern that day was that I not get away with not paying applicable taxes but I can't help but wonder if today the rosewood trade restrictions would also be an issue.
Well said bass12