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davidcyclebackBob, you are expressing proper etiquette, and of course most would follow your lead when composing a business letter. There's nothing inherently offensive about "Hey, Dude," but, as you point out, the phrase might not be prudent at the top of a business proposal. When you're a company trying to get back on it's feet, its prudent to appear formal and business-like, rather than casual. If a company is selling surf boards out of Santa Cruz, the phrase might be appropriate.
Though, I would classify "Hey Dude" as more of a curious greeting than an offense. If it came in a letter from an estate planner or stock broker, I would wonder.
In writing business letters or emails, I have three rules:
1) Use the recipient's name.
2) If you use the recipient's name, make sure you use his correct name. When writing to John, saying "Hello John," goes over nicely. Saying "Hello Jim," not so nicely.
3) If you aren't sure of the recipient's name, don't use his name.
Some other assorted rules I've accumulated for when dealing with potential business customers:
9) With a potential customer named James, don't assume he goes by Jimbo Baby.
11) If his last name is Manson, don't ask if he's related to Charles Manson. Same goes with Marilyn Manson.
14) Don't say that someone's daughter in the photo looks like a skank.
27) Don't wipe your shoes on his cat.