Should a waitress upsell a customer without warning when given an ambiguous order?

At the restaurant where I work, we sell wine. Customers often ask for "just a red/white wine, whatever." Many servers take that as an opportunity to sell them the most expensive wine. Is that moral and/or wise? (Personally, if the customer gives me an order like that, I suggest a few options, usually the house wine and some more mid-range brands.) Should we tell the customer what wine we're selling them and its price? Should we give them the cheapest or house wine? Or should we sell them the most expensive wine, since that will generate the largest tip?