To seal or not to seal, that is the question.
This is very much a personal issue for each of us. There appears to be conflicting information regarding both sides of this issue.
From my point of view, I tend towards the conservative approach when it comes to bonsai care. I like to use cut pastes and sealants on my trees and have personally done experiments regarding the effectiveness of these products.
I have found that there are some varieties of plant material that respond well to these treatments and some that don't. The type of trees you grow, the environment in which they are grown, the density of the wood, and the varieties of wood damaging critters in your area all can play a part in your choices.
For myself, I live in an area with high humidity and high temperatures most of the year. These conditions tend to promote rot and decay in unprotect cuts. Especially on trees that are not native to my area. Thus, my concervative approach.
With one species of plant material in particular, the wistera, not using a sealant will invite all forms of rot and decay to slowly destroy your work. Wisteria require that all cuts be throughly sealed and monitored on a yearly basis if you wish to maintain the integrity of the plant. Even a small cut that is left unsealed will introduce rot into the interior of the tree in a short amount of time.
REgards,
Paul