I plan to paint the deck. Also, my two benches each have eight legs of 4 x 4 cedar lumber. Quarter-inch splits are appearing in seven of them. If you do not paint, then leave the holes open. You can't stop the posts from splitting.
Any solid wood can split, indoors or out.
The bigger the wood mass, the more the wood will split, due to expanding and contracting with gain and loss of the moisture. The only thing to fill the splits with, and I would suggest it not be done, is caulking compound of the color of the wood. This caulking will expand and contract with the wood and thus will stay in place and not break up. Q. I ran across an old House Doctor clipping that described a plastic pencil to fill hairline cracks in plaster. Hardware store people look blank when I ask.