What grade of carrier oils do I use when formulating?

In general, cosmetic grade carrier oils should be purchased and used in your formulations. Cosmetic grade carrier oils are treated with the objective of making cosmetics/skincare, and while they are technically lower grade than food grade, it's not just because the oil is lower grade (or lesser quality). Nut oils, for example, are frequently roasted for use in food oil but aren't in cosmetic grade oils. This results in a stronger flavor and aroma, which is great in cooking but not so much in cosmetics. Carrier oils for cosmetic use are nearly always less expensive than those for food use.