Always. I always use accents and voices, non-verbal gestures with my hands, and facial expressions as well. This not only adds flavor, but can send a message to the party as to whether or not the situation is dire, light-hearted, etc...
I usually take notes on what was said. That usually jars me back "into character", as long as it was in recent memory (3 - 4 sessions ago).
RE: Scottish Dwarves. Dunno. I have often fallen into that myself, though you would think they would have Scandinavian accent. I recently read "Three Heart, Three Lions" by Poul Anderson. This book is at the top of the suggested reading list in the back of the 1E DMG. The list of D&Disms in that book is long: regnerating green rubbery trolls, swanmays, law vs. chaos, etc.. One of the main characters is Hugi, a dwarf (though actually more like a 1E D&D Gnome, as he was statted in Dragon back in the day). Hugi, indeed all of the Fey, has a Scottish accent. Perhaps a generation of budding DMs read this book and decided Dwarves should have a Scottish accent.
If you haven't read it, I recommend it. It's more of a fairy tale than epic fantasy, and a quick, light read. It's quite enlightening to read some of the fiction that the game was inspired by, if not based on.