How tight should I scribe fillers against a log wall?

Your buddy is right about the logs still moving. Fifteen years is enough for most of the big settling to be done, but logs swell and shrink with the seasons forever. It's not ussually a huge amount, maybe a sixteenth or so side to side, but enough to bind a dead-tight scribe. I built cabinets into a log place about ten years back and scribed a couple of fillers snug against the grain, and by the second winter one of them was visibly pinched where the log swelled. Nothing cracked, but it quit looking tight. So my take is scribe to a light fit, then back it off a hair. A sixteenth reveal is fine if you're filling it with a paintable caulk that stays flexible. I used a latex caulk with a little silicone in it, tinted close to the log color, and it's held up fine. Don't use anything rigid, and don't caulk it so heavy it looks like a snake. A tight line of caulk in a scribe gap basicly disappears. On that sloped spot with the inch gap tapering to nothing, the scribe line alone won't handle it. The scribe only follows the contour of one log face, it doesn't take care of a taper running along the length. Rip a tapered strip to fill most of that gap first, glue and pin it to the filler, then do your scribing on the whole assembly as one piece. Scribing a filler that's allready got its taper in it comes out way cleaner than trying to make one skinny piece do both jobs. Use the same stock as the filler so it moves with it and takes stain the same. One other thing that helped me: leave the fillers a little long at the top and bottom til the fit is dialed in, then trim final. Gives you room to sneak up on the line instead of chasing it. And check your fit in the afternoon rather than first thing in the morning. Wood and the cabin both move with the humidity through the day, and a fit that's perfect at 7am can be rubbing by dinner.

When you are working with irregular logs, it is just as important to think about how the cabinet itself is secured. If you bolt the entire base unit directly into the log wall, you are giving the wood massive torque when it expands and shrinks. Most folks make that mistake. I always build a slight recess, usually an inch or so, where the base sits back from the main log surface. This lets the filler absorb all the seasonal movement without the structural frame being stressed. It keeps the whole job from twisting over time.

Also, since you are scribing, pay attention to how the log grain runs. Even if the fit looks fine when you measure, the direction of the grain dictates how it will settle or swell under load. Using a quality wood adhesive and ensuring the grain is facing the right way under that caulk is the key to making that repair last more than a few winters.

Not buying the check-it-in-the-afternoon advice from the last poster. Inside a finished cabin the humidity barely moves over a few hours, so a fit that rubs by dinner has more to do with the fit being too tight to begin with than with time of day. I chased my tail like that on a built-in once and finally took an extra pass off the scribe line and the problem vanished morning and night. The seasonal swing is the real number to plan around, and that's already covered by leaving the reveal. Save yourself the trips back and forth.