I'm currently doing one with overdyed floss and I haven't noticed what you're experiencing. I go left to right and then next row right to left. I'm only using one strand of floss though. Now I'm going to examine my piece and see if there is the same "ridged" appearance that I just haven't seen before now. Do I thank you or growl at you? :)
Liz

Could the ridged appearance be caused by a too tight tension? Change in floss? :)

How many strands of floss are you using?
Also, when you are coming back in the next row going from Right to Left, are you stitching your X’s using the same holes in the same order. Example: I stitch my first stitch from lower left to upper right, ( / ) then stitch my
Other stitch ( \ ) from lower right to upper left and I do follow this exact same way on EVERY stitch no matter which row or X I am stitching. Like VCESS has also stated, it may be your tension when you are going one way versus the other direction. Do you feel it is more difficult stitching right to left? I’m a ‘lefty’ stitcher and it is more awkward for me to be stitching from right to left, which may be affecting my tension.
Also, I have gotten that ridge even using one strand of floss but the ridge seems to ‘flatten out’. Consider doing
your stitching and when you see a ‘ridge’, do NOT ‘frog’ the ridge out, stop stitching, put your stitching away for the day and come back the next day and examine the ridge area again the next day. Sounds silly, I know, but it may be the stitches and the fabric need to relax.

I always thought that if a horizontal row of the same color stitches presents itself, I should stitch half of a cross to the right, then return to the left with the other half of the cross stitch. Perhaps I am mistaken? Does the piece finish more evenly if each cross stitch is stitched completely before moving to the next?

I so appreciate all of the insights I have gained from you, my fellow stitchers, on ECS. Thank you!

Imurdoch, you are correct, BUT with overdyed floss there is a variation in shading so you need to cross and complete each before going to next stitch. :)

You learn something new every day :) .. I have never worked with overdyed thread before, so had no idea you needed to finish one 'x' at a time. Thanks for the information! - Andrea

There are two methods of cross stitch...Danish method & English method. Danish method is the single completed stitch, performed to the left, to the right, or down--NEVER up...popular with "parkers". :)

Thank you all so much for your help! VCESS and Bermuda, you may have a point about the tension. This project is on 16-count, and I usually work with 14-count, so maybe I'm overcompensating somehow. I am using two strands, and I am making each X exactly as Bermuda described. I only wish I had read your advice sooner - I just frogged about 50 stitches! I can't help it - when I looked closer at the stitching, I was not at all satisfied with my work. So, I'm trying it again... Liz, please don't growl too loudly at me! :)

VCESS - have you noticed a difference in the project result with regard to the English versus the Danish method?

Thanks for your expertise! :)

You don't have to complete each "x" before going on, when stitching with overdyes ! You MUST do it with variegated overdyes or even with variegated DMC floss!

Why? Because if you don't, it will come out looking like you deliberately striped it in light and dark!