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


Yes. WFCM Section 3.2.6.1 requires ridge straps, but provides an exception for collar ties, "Ridge straps are not required when collar ties (collar beams) of nominal 1x6 or 2x4 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:
http://www.jlconline.com/roofing/conventional-roof-framing-a-codes-eye-view_o.aspx