When our loft conversion was done they were supposed to fit those in the
several places where multiple joints were bolted next to each other. They
didn't bother and by the time I noticed and complained it was too late to
take everyting out (or maybe they just lied and said they'd fitted them -
I
can't exactly/would rather not remember). And they didn't do the nuts up
more than finger-tight either. And being coach bolts with only a certian
amount of thread they had to fit big piles of washers under the head and
nut
so that they didn't run out of thread when "tightening" them - think they
only had one length of coach bolts and couldn't be bothered to get
correct-length ones.

Anyway it hasn't all fallen down yet. Hopefully if you do the job better
than that then yours will be fine.

Regards,
Simon.

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.