There's a Fredette guide on my '05 MC and my buddy's '04 DE and they seem fine.
And here are some musings on the subject from a relative newbie...
As far as chain tension, my technique (which I found in a Husqvarna manual) is to put a ratchet strap over the back fender and compress the rear suspension until the countershaft, swing pivot and rear axle are in a straight line. By geometry this is the longest point where the chain would be tightest. I leave a "slight amount" of slack at this point. After you do this once you get a feel for how much slack you'd actually need with the "two-finger" method.
Another way to check alignment is to put a straightedge against the flat on the rear sprocket and aim it at the front. (easier with the chain off) Once aligned this way, for future reference, you can check to see if your marks are "on" and also verify whether the tabs for the chain adjusters are equidistant from the axle. IMHO the marks are "close enough" for a dirt bike chain... I'd think it would have to be way far off to actually cause a derailment.
I dumped mine in wet sand one time and so much sand packed into the chain that it felt like the tranny was locked. I tried for "awhile" and couldn't rock the thing into neutral... I thought I had damaged a shift fork or something. Anyway I finally started it in gear and worked the clutch enough to get moving and it freed right up. I wonder if on that uphill mud section you just got lucky and picked up exactly the wrong rock or enough mud to kick your chain? Wouldn't help to have your chain set too loosely in that scenario... I will say that it looks like it would be damn hard for the chain to jump with that Fredette guide in place.
