If a rafter strap is used at the peak, can collar ties be eliminated?

Yes. WFCM Section requires ridge straps, but provides an exception for collar ties: “Ridge straps are not required when collar ties (collar beams) of nominal 1×6 or 2×4 lumber are located in the upper third of the attic space and attached to rafters in accordance with Table A-3.6.”

See the following article for more information: