Apple's official documentation on Photo Stream states:
My Photo Stream stores the last 30 days of photos taken with your iOS device or imported from a camera to your Mac or Windows computer, and automatically pushes them over any available Wi-Fi or Ethernet connection to each of the devices that you set up to work with iCloud.
For example, if you take a photo on your iPhone (or save one to your Camera Roll), iCloud automatically stores the photo and pushes it to Photo Stream on your iPad, iPod touch, and other Mac or Windows computers.
What's not clear to me is this: does iPhoto actually have to be open in order for Photo Stream photos to be pulled down to my Mac?
The reason why I ask is because whenever I open iPhoto, there's a noticeable delay before I see any new Photo Stream photos appear, which suggests that those photos are being downloaded right then.
If this is indeed the case, given that Photo Stream only stores the last 30 days' worth of photos, does that mean that I'll start losing photos if I go longer than 30 days without opening iPhoto?