Rainy days are indoor days at my house.
My dogs hate it, when they need to go to the toilet outside they race out, wee as fast as they can and then race back inside!
There are some great waterproof dog coats out there, and dare I say it.... you can even buy rain hats for them
If your dog doesn't mind or even enjoys walking in the rain then go for it. But just be careful on cold days and make sure you dry your dog thoroughly when you get home or have a heater going inside.