In order to preserve moisture, conventional (non-organic) and organic cucumbers could be coated with a wax. The difference is that the organic cucumbers must be coated with non-synthetic wax, and they cannot contain any chemicals prohibited by the USDA under organic labeling laws. Conventionally grown cukes can use synthetic waxes and may contain a number of pesticides on the skin, as well as undesirable chemicals in the wax.
As far as wax goes, World's Healthiest Foods recommends peeling all conventional cukes and leaving the skin of organic cucumbers in tact for its nutrient quality.
English cucumbers - the long, skinny kind, tend to be unwaxed and wrapped in plastic - organic or not. here are two organizations who have growing databases of pesticides used on cucumbers and other conventionally raised fruits and vegetables:
1. The Environmental Working Group's Shopping List: Conventional cucumbers are in the top 10 for high pesticide use, and is in the EWG Dirty Dozen list, designed to help shoppers quickly decide which produce should purchased with a USDA organic label. They recommend that the cucumbers you buy be organic.
2. What's on My Food? a Database from the Pesticide Action Network: 35 Pesticide Residues Found by the USDA Pesticide Data Program for cucumbers.
Given this data, The Tender Palate uses organic cucumbers, and does not peel them, unless we want to for the sake of a recipe!