If your primary fermentation (sugar to alcohol) is complete you shouldn’t have to add any potassium sorbate to your wine. In fact, adding sorbate to wine after performing malolactic fermentation (ML) can cause an unwanted effect in the finished wine — potassium sorbate reacts negatively with lactic bacteria and results in a geranium-like off-odor. The alternative to using sorbate when you plan to use an ML in your wines if you are worried about refermentation is to stabilize the malolactic-fermented wine with sulfite followed by a fine filtration, then monitor the wine during bulk aging to ensure that fermentation does not restart.
I am also personally not a big potassium sorbate fan as it sometimes can have a pineapple type off-aroma that I don’t find attractive. Similarly, I’m not really into adding things to my wine when I don’t have to. Will ML fermentation affect the flavor of your wine? Definitely, and that is one of the main reasons why winemakers do it. ML fermentation always de-acidifies the wine a little bit and sometimes, depending on the strain of ML bacteria you use and the components in your initial wine, you can get some cheesy or buttery notes in the aroma and a general rounding-out of the flavor profile and mouthfeel. I find all of these effects to be a positive, especially in red wines, and perform MLF on all my red wines, as do most of the commercial winemakers I know. Good luck with your wine and new procedure!