Something that comes up every now and then as a slight niggle: I can't seem to make marker positions sample-accurate. If you zoom right in and place markers, they tend to be to an approximation in musical time. Is there a way of placing them at sample-level precision?

I have done this and there is a screen shot in this post of it. I was on the search for a single moved sample (done in Audacity) and after I finally found it, I put in a marker. I do not recall having to adjust any settings, and was zoomed in as far as the pic shows when I marked it.
 
Although now that I look at that again, I think I see your point. Perhaps the resolution is simply ticks and I got "lucky" with placement.

IIRC, your options are SMPTE (i.e. frame-level precision) and M:B:T (tick-level precision). Depending on tempo and sample rate, ticks should get you to ±10-15 samples.
 
EDIT: In Michaels' screenshot linked above, it appears to me that the referenced sample location just happened to be close to a whole tick.

Ah, I see. Thanks. Would be nice to be able to get sample accurate, but I suppose it's not something that comes up a lot for many people.