The prop guards don't weigh much and don't really even come into the equation when talking about flight times.
I would personally recommend them. If not to protect your propellers themselves, then protect yourself.
Check out our blog post that shows some propeller induced damage.
The props are roughly $8 each and the guards are $6 each. It is not uncommon to ruin a couple props at a time the event of a crash. My record is 3. That's $24 right there. Could have put the guards on and come out even... A bad crash can break the guards too though so it is best to learn to fly well before getting into any tight situations.
The screws are a one time deal and it should only take a couple minutes. I think the whole rig looks better with the guards on.
They make a nice way to carry it around too.
Some people find that they add instability and remove them right after putting them on.