Implant level or abutment level impression?

The question of accuracy is not relevant to implant or abutment level unless you are using angled abutments to correct for intentionally off-axial placement. The choice of impressing at either level is dependent upon the type of abutment you wish to use. If one-piece, solid abutments, then you place them and impress. If two-piece or custom (CAD/CAM), then impress at implant level. Yes, you should have a bite registration otherwise the lab will have no way to mount the casts and fabricate the restorations.

There are several other factors to consider: 1. The rotational and vertical discrepancy of the connection : Most operators do not take these into consideration, the first causes a problem because the abutment can be secured back in a different rotational position. The second because the abutment may be higher or lower depending on how much the impression device then the abutment is tightened down. These errors would advocated to placing abutments, tightening them down and then impressing / calling the abutments. 2. Impression device : There is a tendency to use PEEK components for scanning. This material can be relatively "soft" within a connection, so that it is easy to build in a rotation discrepancy 3. Lever Arm : The deeper the implant the longer the lever arm so orientation problems will be larger. Conclusion : a preliminary implant level impression can get reasonable accuracy, enough to construct abutments. When the abutments are seated and torqued down then they will be at their final position. This might be the best foundation to take a final scan / impression to construct the final splinted reconstruction. Any problems which occur will be at the abutment level, not at the implant level.

With multiple splinted abutments where parallelism is required an implant level impression will make abutment selection and preparation easier. When the implants are to be restored as individual separated teeth and the alignment is good abutment level impressions can work good.

You said, "the implants would be supporting free end saddle distal extensions". That sounds like removable partials. Is that the plan for final prostheses?