I like to hose mine in the summer, they absolutely love it when it's hot out and they've had a good workout or trail ride. It isn't a black and white issue, though. As long as your mare is well cooled out, and has stopped sweating, you're probably ok health wise. It's the cooling out that's the issue, and putting a hot horse away sweating isn't a good idea. Winter I don't get to do much riding, but my advice there is to just be sure that your mare is well cooled out and dry before putting her away, perhaps with a fleece sheet. If she's shaggy and is hard to cool, and you do a lot of cold weather riding, perhaps clipping and blanketing is a good option for you.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown