I figured that the best way to ship 2D art for multiple resolutions is to instruct my artist to create the fullscreen background images sized for the highest resolution and then scale them down during the loading stage.
To scale them, during load, I setup a render target sized to the current resolution, SpriteBatch.Draw the assets scaled to the current resolution and then use that render target throughout the game. The render target serves as a SpriteSheet throughout the game. I only do this once (in LoadContent) and not every frame.
My question is whether or not there is a memory/performance penalty for using render targets instead of whatever the ContentManager would've set the textures as. Is there a meaningful way to measure this? Or is there a better way of doing this altogether?