The travel reimbursement policy I am requred to follow in order to secure reimbursement calls for me to secure the lowest fare for the routing.

It should be noted, however, that "lowest" is a somewhat flexible term.

For example, there are times when I can delay my return flight to the following day and, even subtracting the overnight hotel stay, the cost can be several hundred dollars less than fly in-fly out.

Also, I am roughly equidistant between MKE and ORD. Occasionally (with much less frequency now) fares are cheaper from ORD. However, higher parking costs, additional road mileage, food costs, tolls and the additional time it takes to get from the parking lot to the terminals at ORD can make a slightly higher priced ticket from MKE actually cheaper. In such cases MKE wins out.

If cost was never a concern, YX would get my business all the time but, even as a stockholder, they do not. Generally, NW or its partners has the lowest equivalent "real" cost. Occasionally another airline's fare may be a few dollars cheaper up front but that may be easily offset by other costs. (Ironically, one thing that may happen is that CO may sell the same seat on the same NW operated flights for less than NW! In that case I would book on CO but bill the miles to NW.)