Letheing pets at a certain level is just a common thing people do to use less scrolls at a higher level, but no matter when you start letheing both will be equal stats points if full washed and you have them both matching point base of what you scroll them to (agi, con, etc). Some pets have different starter builds (when caught a point can be placed 1 higher or lower then another one beign caught) so take note of the builds and pick the higher one of the stat base you chose.