I solved a few images with a tweak order from 0 to 5.
Below the text are the results.
The first column is the degree of tweak order. The second and third are mean separation(measured as the average on-sky separation between field and index stars) and standard deviations in arcsec, respectively:
tw=0 7.184726'' 3.835836'' 2.213243
tw=1 10.922670 5.403251 2.722024
tw=2 7.184726 3.835836 2.213243
tw=3 2.057162 1.731583 3.660477
tw=4 1.843406 1.531140 3.946095
tw=5 1.446841 1.407375 14.701140
So, increasing the tweak order gives better and better results.
As I mentioned before, I am interested in one particular pixel at the very edge of the image. I am comparing RADec of that pixel with a RADec of a known star near to it.
The last column gives results for that separation, in arcminutes. So, by increasing tweak order, the separation between those two points also increases, but if solving is better(more accurate as we can conclude from the separation results), I am expecting it to decrease.