Yeah placeholders are key because it allows you prototype designs. The design of a game should be the top priority because it defines whether it's going to be fun or not. Once it's functional and you know it works well, you can start to dress it up and make it look pretty.
Even professional games do this with block mesh maps before the artists come in go nuts. If you don't want everything to be blocky colors and roughed out forms you can try something more concrete like opengameart.org.
In my own experiences I tend to make whatever's needed at the time. Though back when I had less experience I would skip to a particularly cool area and start making graphics for it but there's kind of no point because you're not going to get to that point in the game for a while and you will probably be a better artist once you do. Although I have also learned to let inspiration dictate, if you are itching and inspired to work on a certain part, then do it before that feeling leaves you.
Which brings me to my next point. Reaching that final polish is not something that is achieved once over. Reduxing is key. Create an area, and then come back to it later when you have accumulated more experience and then fix the hell out of it. After several iterations it starts to shape up and take form.