Thanks for the input.
I use a Beseler alignment tool, the one that has two bubble levels at right angles. The lens board, negative stage and baseboard are all in alignment. Then I place the easel on the baseboard. The center of the easel is closer to the lens than the 4 corners so it is crowned slightly. I would not have noticed this by looking. Likewise the effect is modest at smaller enlargement ratios. However, on a 16x20 print (4x5 negative) the 4 corners are unsharp. The top corners of the easel are worse than the others.
That is interesting that the vacuum easel might even out when the vacuum is activated. But I really don't want to use it (even though I bought it). Why would someone prefer a vacuum easel for baseboard enlarging anyway?
I don't think it is lens field curvature because I can measure it on the easel with the levels. The lens is a Schneider Apo-Componon 150 mm. I think I have had this issue all along but to a minor degree. Must be something in the are this time of year.
For the reasons mentioned, stopping the lens down does not help much.