Nutritionally, rolled oats and steel cut oats are fairly close on the same glycemic scale with steel cut being a little lower. The issue is steel cut oats will be much chewier and may take more liquid than rolled oats for good overnight oats. If you're making overnight oats, go with regular rolled oats. If it's a big concern, you might be better off cooking steel cut oats, cooling and refrigerating them, then adding your preferred toppings.