The thing is, water in the 50s is not dangerous to the plants at all. It's just not the optimal temperature for the plants to make their maximum growth. That being said, you can probably more than make up for any impact of water temperature just by fertilizing the plants lightly.
If you're really concerned, just leave a 5-gallon bucket of water inside your house, tucked away in a corner or bathroom somewhere. Overall, however, water temperature shouldn't be a big worry. All you will lose is possibly a couple of inches of growth over an entire winter. If you simply ensure your media is draining properly, and I'm sure it probably is, you have done the biggest and most important service to your Neps.
Constantly cold and stagnant water, however, is an optimal environment for many pathogens. Also, watering early in the morning means your soil has an entire day to warm back up--if you water at 6PM and it's dark, the plants won't drink any water all night long. They'll just sit in cold water overnight and that's just not good over the long term because it can lead to disease. If you start getting things like mildew and botrytis, it's a big warning sign that things are too cool and stuffy. I come into work all winter and immediate water what needs it, and I set all drip irrigation to begin when the sun first pokes out.
If you are using 50-degree-ish water, just ensure it doesn't sit anywhere too long, and definitely don't use trays in the winter unless your plants are 10 feet long and drinking like sea monsters.