I've seen a similar question before with mixed answers. I have a mixture in my house simply because we added on and wanted to upgrade the trim. Most of the house has 6" poplar baseboard in a medium stain and similarly stained door and window trim. However, I painted the door trim in the original part of the house as it was part of the prehung doors and couldn't be removed without destroying the door jams, but replaced the baseboard with the medium stained poplar. On the other hand, my husband refused to replace the baseboard in our BR, so it is the original baseboard, painted white but the window trim is stained. It sounds bad, but really it isn't. Here you can see the painted door jams and baseboard, and stained trim.