I have found it makes a bigger difference in a underpowered mower, such as some electrics. If you have enough HP it compensates that you can get a decent cut on a duller blade. Once even put on backwards and it still seemed to work OK.
People pay others to sharpen thier mower blades!?
I have a DeWalt 4.5 inch soft pad grinder that sharpens the livin’ shit outta a mower blade!
Yes, it makes a difference.
My local Sears did. Then they started just replacing blades. Then they only sold the blades for you to replace yourself. Not sure if they do any of that at all anymore. I don’t even know if they still have a repair storefront.
The general rule of thumb is to sharpen the blades three times during the growing season.
If you haven’t sharped in a long time (or never), you could mow part of your lawn, sharpen the blades, then mow the rest. I suggest that if you can’t tell the difference, you aren’t paying attention.
With the string type weed whacker, you really don’t have much interest in the health or appearance of the weeds – the goal is to remove them.