Consider this post on snow-delay.
#25. Parent correspondence: how often do YOU initiate it?
Confession: I don't do as much as I want to.
At the beginning of the year, I send a letter home introducing myself and providing my (school) email address. Students return the bottom of the letter signed by parents, whose signature agrees that they have seen both my syllabus and the letter. I also include the web address to my frequently-updated website in that letter. That way, at the very beginning of the year, I give them the power to contact me. And they do. I haven't had too many negative interactions with parents, honestly. I like to think that it's because from the very start, I am upfront about ways to contact me and making myself available. So, I guess you could describe my approach as slightly passive-aggressive.
After that initial contact of "ball's in your court," I usually don't initiate unless I'm really concerned about a kid: multiple instances of acting out (rare), poor grades (often), poor assignment/make-up work return rate (often), or attendance concerns (very frequent fall 2009). One of my goals on my never-ending list of To Be A Better Teacher I Will... is to write a positive note or make a positive call home at the end of every day for at least ONE student. That way, I can end MY day with positive vibes, and also eventually "get" those middle-of-the-road kids who get left out- not struggling, but also not super-stars.