I did some a few months ago and planned to use the
star-washers-or-whatever-they're-called. But they kept the joists apart
however much I tightened the coach bolts. I thought the joints were more
solid without them so didn't use them in the end.

I assumed the gap was why you did use the star washers so that the joists
can expand and contract symmetrically.

No I don't think so; I believe that ideally the star washers are supposed to
bite deeply into the timber on both sides, leaving little or no gap. The
joists wouldn't need to expand much side to side anyway. My joists might
well have been strong enough if I'd used the washers, but I was happier with
the timbers tight up against one another. Maybe I have unusually hard timber
for my joists...