If the mold drops bullets that are
round, at the proper diameter, with the alloy you are using, then there's no need to size.
But, it rarely happens.
It's even more rare for a
Lee mold to drop bullets of the proper size. (And forget about them being round.)
The only mold I own, that drops bullets that can be shot as-cast, was a very expensive beast to develop. I went through three different molds ($145 / pop), and about 5 different revisions that never got cut, before the final single-cavity mold gave me a bullet close enough to the goal. It's a bore-rider design for a single shot rifle, using a hard (high antimony) alloy, because I'm pushing the bullets to 1,900-2,200 fps. All I have to do is gas-check, lube, and load. However, seating the gas checks is easier if I just run the lubed bullets through a Lee sizer. So, I still perform the extra step, even though it's just crimping the gas check, and taking a couple ten thousandths off the body diameter as a minor byproduct.
But...
My other molds don't afford that luxury.
The Lee molds are all over the place. Some of the cavities drop bullets that are as much as 0.006" over the target diameter; while other cavities drop bullets only 0.001" over the target diameter. Pretty close to ZERO are actually round; some are even out-of-round by 0.004" to 0.006".
Lee molds are a roll of the dice. You never know what will turn up.
They work, but you should always expect to have to size the bullets.
Even my NOE molds (known for fairly tight tolerances) have cavities that drop different diameters.
For example:
I have three different versions of the NOE 314316 115 gr FN 2-cavity molds. They're all from the same production run, and each cavity in each mold drops a different size bullet. I can't shoot anything over .314" in my revolvers (due to chamber size), but most of the cavities at drop .315" or .3155" with the alloy I use (with one outlier at just a hair over .316"). So... everything for the revolvers gets sized in a Lee push-through sizer.
On the other hand, a slight tweak to the alloy gives me a few tenths more diameter, so I can shoot
unsized .3155" to .316+" bullets in my Mosin-Nagant.
For all of my bullets, I dip-lube or pan-lube. The deciding factor, is the size of the bullet. If I can hold it securely in my fingers, and dip all of the lube grooves without getting lube on my fingers, they get dipped. (Examples: the bore-rider,
Lee 430-310-RF)
Otherwise, they get poured (pan-lubed). (Examples:
Lee 429-200-RF,
NOE 314316)