Yes, Mow with it in the lock down position, this will allow for the deck to float, should the deck wheels ride on something high. To check it out,put the lever in the lock down position and then get a helper and each bend over and lift up the mower deck from each side and you will see that it floats.
The real help in using the lift in the lock down position is when you are using a front blade for snow plowing.
For instance, when you are plowing into a snow bank and the blade rises up the blade will not ride all the way up and lock in the up lift position and break something.
By leaving the blade in the lock down postion, it allows it to float.
Less likely to break something that way under the tractor.