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"In theory, the XS should be straight"
No, I would say in theory the XS should be perpendicular to the direction of the flow. Flow perpendicular to the cross section is the assumption of one-dimensional programs like RAS.
For an "S" shaped river, sometimes you have to make an arbritrary XS shape just to keep the XSs from crossing each other.
I usually draw the channel part of the XS as a straight line. I adjust the shape of the XS in the overbank to try and keep the flow perpendicular to the XS section line. Ideally, the XS should be whatever shape is required to keep all the flow lines perpendicular, but it depends on how much time I want to spend messing around with the XS and whether it seems hydraulically relevant for the given data set.
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