Hold the phone here. Which bars are we talking about?

If we are talking about the ones on the plastic standoffs up by the main breaker, they are electrically equivalent and you can connect the neutrals to whichever one you want. If the bonding screw is installed (i.e., this is the service disconnect), you can also land your grounds there.

If you are talking a bar lower down by the breakers that is screwed directly to the case that is ALWAYS a ground bar. You can not land neutrals there.

QO panels come with only the combined neutral/ground bars on the plastic standoffs. There's a screw that bonds this to the case if this is a main panel. If you want to use this as a subpanel, then you take that screw out and you have to mount a seperately sold ground bar to the case. There are tapped, raised holes for this on both sides. You can install one on either side or both.

ronnatalie, Thanks for the reply. This is the entrance (service disconnect) panel and the bars in question are definitely neutrals/grounds. I know what you are saying about dedicated grounding bars to the case (I had to put one in my incorrectly wired out building to separate the neutrals from grounds), but the entrance panel has no separate ground bars. And being the main breaker the bonding screw is in place.
Thank you again for the answer.

Land the neutrals and grounds for each circuit adjacent to the breaker the hot is landed on.

If the breaker is very near the cable entrance to the panel, leave some extra wire folded back on itself.

While some of the jobs look very 'neat' with direct runs of minimum length, they turn into a nightmare as soon as anything needs to be moved.