If you buy a random thing off lets say etsy i wouldnt say tagging the store is necessary but hey it sure is nice! If you like the piece wouldnt you want to help support the artist with getting them business?

But for commissions, specifically cosplay commissions, if you arent tagging the artist... One it looks like youre claiming the work is yours effectively lying to your audience And two you arent doing your part to help your community thrive.