In Chinese culture, politeness is never too much only except for between really intimate friends or lovers. Especially when getting along with an elderly person, it's a good idea to keep being formal and polite until you're completely certain that it's not necessary.
This link explains the conventions very well. To cite the essence of it,
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”。
When in doubt, be as polite as possible. When I feel obligated by the culture, I would simply use formulaic 您好 as 问候语 and 此致敬礼 as 结尾.
Edit:
Adding translation for the citation with my editorial in parenthesis. The translation is done in a kinda verbatim way in order to preserve the sense of the culture, although it may sound unnatural in English.
2 . Greetings
Greetings should be placed on the second line ( the first line is a title) with an indentation of two spaces.
Use polite language to make the recipient feel warm and respected.
For elderly recipients, send regards to health; for middle-aged people, career and family; for a young person, love life (only appropriate if the sender is an elder to the recipient) and studying; for children, health and striving.
4 . Ending
To show politeness, use some blessing words based on who the recipient is (social status and relationship to the sender).
A general one is “此致”“敬礼” (usage similar to "best regards" in English, literal translation should be "hereby, salute"). The format convention is to put "此致" on a new line with an indentation of two spaces and "敬礼" on the next line without indentation.
People usually say "祝您健康长寿” (wish you health and longevity) to elderly people, "祝工作顺利" (wish you a successful career) to friends and "祝学习进步" (wish you good progress in studies) to the younger generation.
