This one is tricky. I've known some atheists to find the waldorf approach totally repellent and others (my younger brother, for example, who went to a waldorf school for several years) to be totally okay with it. I think you might want to try reading a book about waldorf education to get a bit of the flavor around the "spiritual" side of the education to see if it bugs you or not. It would be hard to find out enough about this aspect of waldorf by just visiting a school.
I went to a waldorf school for a couple of years as a teenager, and I can't ever remember being preached at. My daughter attended for 13 years. She is, I think by nature, very open to the sort of cosmic stuff that is a background to waldorf, so she was happy with the general tone of the education. For example, in 5th grade, her teacher told some of the myths from India. My daughter came home and told me that now she understood how it worked, people were born again and again. In other words reincarnation. Now, I'm sure her teacher just presented the story, leaving the children free to take it in and make of it what they would. For my daughter it was the obvious presentation of "how it works" but for other children in the class it may have simply been another great story. I didn't hear from the other children in the class.
My granddaughter, now in the nursery school at our local waldorf, participated in a lantern walk a few days ago. The preparatory handout described the schedule and asked the parents not to break the mood of the story and the walk, which was supposed to inspire feelings of quiet reverence in the children. For some parents, something like this could be...not what they want.
A couple of examples, hope this helps.
PS Just wanted to add, regarding the reincarnation story, if I had not been positive about her belief in reincarnation my daughter probably would have moved it to the "just a story" column. Parents have a lot of influence over children's long-term structure of beliefs. On the other hand, if everything a child experiences at school is going to be mocked or belittled at home (something I got a bit of from my parents who were fairly hostile towards much of the content I got from public school), it definitely undermines a child's respect for their teachers and the seriousness with which they take up their school work.