For gambit/EG events the slayers get like 10-20% better if you lvl them up to 100. IE the Sr beta went from 1.3 bonus to 1.5 bonus for maxing it out. For Shards though the level doesn't matter for slayers, its always 1/2/3 bonus shards.
Also the bonus will scale as you level them up so even if you cant max them right away you should still use wilds when as soon as you can.