I've always heard that you should use placeholder graphics. You can always change things up later as you finalize your game. Personally, I want to try and use invisible tiles with my own custom painted backgrounds. I used RPG Maker 2k and 2k3 years and years ago when I was a kid (now in my late 20's) and I recall being able to easily do that, so I hope it's still possible with VX Ace. I'd still use some tile assets like trees, random objects and such. I've grown as an artist, but when it comes to anything crazy like programming, I can't do it at all which is why I still prefer to use this rather than a more advanced game engine.
But before i start painting my backgrounds, I'll be using the tilesets as a guide/rough draft. Even then, the tiles don't look bad. Like someone else mentioned earlier, you can use lighting changes and shading to set the mood. As a low level art student, i can tell you, lighting is everything whether it's art, video games, photography, home decoration, movies, etc etc. Lighting can make pretty people ugly and vis versa. So being able to manipulate it will make any decent tileset work for your game.