When you first import photos, you are presented with a dialog that offers to delete the items on the iPhone (or iPad). This is the same in iPhoto and Aperture.
There's some sense in rejecting that option. And there are some people who foam at the mouth about how you should
reject that option. However, in downloading some 100,000 photos from several different cameras with many different cards in them, to iPhoto and Aperture, I've never lost a photo because of a failure to download properly. I have
lost some photos because of a total hard drive failure, and some because of user error
OK, moving on...
You can delete anything from your iPhone. And you won't lose anything as long as you've already downloaded the photos to your Mac
Why the italics? Because a relative of mine kept hundreds of photos only on her iPhone. Then her child threw her iPhone into an aquarium.
So I recommend the following:
1. Always download any photos you take with your iPhone onto your Mac. (Because, of course, being a SMUG member in good standing, you have excellent backup habits for your Mac...) (See additional note below.)
2. Never keep photos on your iPhone without downloading them to your Mac. (See #1.)
3. In iPhoto (or Aperture), set up an album with any photos you want on your iPhone. In iPhoto (or Aperture), these are just aliases, so you can add and delete as you like.
4. Sync that album to your iPhone.
5. Never expect your iPhone to be the only storage for any photos you might ever want to keep. (See #1.)
All of my rant above applies to any digital camera, not just an iPhone. While there are many computer files that you can--sometimes with a lot of work--replace, many photos are not replaceable. Guard them with care.