Fertilizing/pruning perennials over the winter?

If they are still flowering, yes. I would check with a local extension office or a trusted greenhouse about the fertilizing. When it gets chilly they may not need as much.

Removing the dead flowers from hydrangeas makes the plant look better through the winter. Hydrangeas flower on new wood, so the added bonus is that you will have better flowers next year.

Always deadhead spent blooms. No need to fertilize during dormant season. What else do you have besides hydrangea? These links will help you out!

http://www.oregonlive.com/hg/index.ssf/2016/07/gorgeous_hydrangeas_and_what_y.html

http://blog.oregonlive.com/homesandgardens/2007/09/hydrangeas_are_what_gardeners.html

http://www.hydrangeashydrangeas.com/planting.html

nope - even in a mild climate, plants need a resting season, so only fertilize plants that go 'in' to active growth during that season - hydrangeas should go quiet for at least a month or two...

and I'm surprised you fertilize, Oregon had pretty rich soil, from what I saw of it, most gardeners only used compost.