An approach that we routinely use to eliminate edge effects, which I believe is what you are describing here, is incubation of our plates at room temperature for 15 minutes post plating prior to movement of cultures to 37 ºC incubator. This as well as ensuring that your medium is at room temperature prior to plating of cells has been shown to be critical in eliminating edge effects. This paper, http://jbx.sagepub.com/content/8/5/566.long (J Biomol Screen. 2003 Oct;8(5):566-70) outlines a comparable approach with extended incubation for 1 hour at room temperature prior to moving cultures to 37 ºC incubator.