Everything I've read on past posts say you should leave them a bit dull, ie not sharpened to a fine edge. Which reminds me, I've got to get mine in the air to do some grinding soon too.
Some say yes some say no,for me it depends on what type/how thick.
I razor sharpen blade,it won't take very long to loose the edge after couple good whacks.
What are you cutting? If I had a lot of sapplings or blackberries (I do) I would leave them dull. Can't keep them sharp anyway, but the dull impact tends to shatter the stem, making it harder for most plants to regrow (saddly not to blackberries). But if you are doing lots of grass, and also aren't concerned about puncturing tires, then sharper is better.
All depends on what you are doing. For rough work, ie, brush, small trees, rough ground, etc. let the blades dull. However, for mowing fields/grasses I keep my blades a little sharper.
New blades aren't sharp at all. Look at the new ones at TSC .