Maybe the driver is a good idea, not just for the wine but also as some wineries are not at all easy to find for an outsider.

I have visited Bodega La Rural on a Saturday and Bodegas Salentein on a Sunday last year. No problem, they had tours a.m. without prior notice. Found La Rural the more interesting one, and enjoyed lunch at the nearby Casa del Campo. The Trumpeter Shiraz-Malbec they produce must be one of the wines with the best price-quality relations in the whole world.

This is right on, signage in Mendoza is very unreliable, and even if you are fluent in Spanish and most locals are very friendly, most people know very little about which winery is located where - so you may end up getting lost and wasting valuable time. Prices for transportation with a driver are more than reasonable, so by all means do hire a vehicle with a driver.

Some additional tips:

If possible, have your schedule set out so you advise the driver in advance where you need to go, ensure he knows the bodegas you will visit.

If possible, have a cell phone and all numbers of the bodegas handy in case you need to call and get directions.

Budget some additional time (10-15 minutes) so that you get to each bodega on time, else you may miss your the guided tour.