Photostream will store the previous 30 days worth of photos automatically. That means, every photo you take on your iOS devices will be sent to Photostream, but only the last 30 days worth of photos will be available there.
If you enable 'Automatic Import' of Photostream in iPhoto (Preferences > Photostream > Automatic Import) then when you launch iPhoto, it will automatically download a copy of any photos that are in your Photostream, and automatically create an Event titled: {MMM YYYY} Photo Stream (eg. "Jul 2012 Photo Stream")
This Event will be automatically kept up to date, as long as iPhoto is open long enough to sync everything required (depends on your connection).
In effect, as long as you open iPhoto once every 30 days and leave it open long enough for your Photostream to be imported, you should never have to manually import a photo from your iOS device at all.
As for removing items from your Camera Roll on your iOS device, as long as you see that they are present in Photostream (or in iPhoto), then it is safe to delete them from your device. (There is no way around this but to do it manually as far as I know.)
Currently, Photostream does not support videos. To import videos, connect your iOS device to iPhoto and import these manually.
As a side note, in order to keep some photos on the device, you can create a smart album in iPhoto along the lines of "Photos taken within the last 30 days" or "Photos rated 4 stars or more", and configure iTunes to sync these to your device. That way, you can be sure that no matter if you clear your Camera Roll on your device, or delete certain photos, you maintain some convenience.
Best Answer
From Apple Support:
Unless you disable "Automatic import" from the options, there is no need to manually copying them as they already are in your library.
Also, adding them to iPhoto albums does not make additional copies of the pictures, just lets you arrange links to the originals in any fashion you like. In fact if you remove a photo from an album:
And neither from the library, for that matter.
This is because iPhoto albums work like labels, instead of being real folders (see an explanation here).
So to answer your question, enabling My Photo Stream on your iOS device and iPhoto, keeping auto import on, should suffice to have one copy of your pictures on your hard drive, given you regularly open iPhoto and have internet available for both (not necessarily concurrently ).
Note that videos are not uploaded via My Photo Stream, and that you may run into trouble if you take more than 1000 pictures or let more than 30 days go between each iPhoto session.
Check Apple's FAQ on the details of this limitations.
I would advise to implement some kind of backup anyway, as this scheme will at the most leave you with one —maybe two copies if you don't delete them from your iOS device. Time Machine is easy and free; for a dedicated picture/video solution check iCloud Photo Library (paid).
This information applies to the Photos app too at the moment of writing.