In Chinese culture, being over polite is never too much only except for between really close friends. Especially when getting along with an elder person, you should keep being formal and polite until you're really really close with that person. So even writing to a person that I'm familiar with, have a good relationship with and maybe hang out together a little bit, I still prefer formal and polite words.
I found this link that explains the conventions very well. To cite the essence,
2 ．问候语
第二行开头空两格写问候语。
运用礼貌语言，使收信人感到亲切，受到尊敬。
长者多问候身体，中年人多问候事业和家庭，青年人多问候爱情和学业，少年儿童多祝愿健康成长。
4 ．结尾
要根据收信人的身份，写表示祝愿的话，以示礼貌。
一般性的祝词“此致”“敬礼”，格式是另起一行空两格写“此致”，“敬礼”下一行顶格写。
给长者的信往往写“祝您健康长寿”，给朋友写“祝工作顺利”，给晚辈写“祝你学习进步”。
My own experience is, when in doubt, be as polite as possible. When I feel obligated (by the culture) to be polite but I don't feel comfortable being too polite with that person, I would use formulaic 您好 as 问候语 and 此致敬礼 as 结尾.
Edit:
Translation of the citation (with my notes in parenthesis) as per ask. I prefer to translate word-by-word with little embellishment, which may sound stiffy but preserves the sense of Chinese culture.
2 . Greetings
Greetings are on the second line (first line is title) with indentation of two spaces.
Use polite language to make the recipient feel warm and respectful.
For senior recipients, send regards to health; for middle-aged person, career and family; for young person, love life (only appropriate if the sender is an elder of the recipient) and studying; for children, health and striving.
4 . Ending
To show politeness, say some blessing words based on the recipient's identity (social position and relationship with the sender).
A general one is “此致”“敬礼” (similar to "best wishes" in English; literal translation is "hereby","salute"). The format convention is, “此致” is on a new line with an indentation of two spaces; “敬礼” is on the next line without any indentation.
People usually use “祝您健康长寿” (wish you healthy and longevity) for seniors, “祝工作顺利” (wish you a successful career) for friends and “祝学习进步” (wish you progress in your studies) for juniors.
