In Season 1, Elliot was on the left (or right side, depending on how you look at it) and Kathy was on the right
In Season 8, Elliot continued to sleep on the left side however this time alone
In season 9, Elliot and Kathy seemed to have switched sides- Elliot on the right and Kathy on the left
In season 10 (one year later), they switched places!
In season 12, they switched again!
I find that rather humorous because in such shows like "Bewitched", "I Love Lucy", "Everybody Loves Raymond" where it shows a married couple in bed they pretty much stick to 1 side during the tenure of the shows. Meanwhile Elliot and Kathy seemed to like 'musical sides'.