When converting CAD to USD on Interactive Brokers Trader Workstation, should the price of USD.CAD be as high or as low as possible?

This is the USD/CAD pair. It represents the price it costs to exchange USD for CAD, currently 1.250x CAD per USD. If you were to buy this trade, you would be buying CAD for USD, and would want as many CAD as you can get for your USD, so would want the limit price as high as possible.

You however, are on the other side of this trade, and want to sell CAD to people who want USD. As a result, you want to get as many USD as possible for you CAD, which means you want this value as low as possible (to use an extreme case, you obviously would prefer to only give up say, 1 CAD for 1 USD if you could here, rather than having to give away 1.25 CAD per USD and paying the extra 25c).

In more practical terms though, the spread here is absolutely minute so not too much to worry about. Just using market orders or a limit at the current sell price is totally fine.