I think polish is probably quite tricky due to its subjective nature and while you are working on a project; you'll probably get better at refining your visuals, audio and so on, but this can lead to odd shifts in quality or constant refinement of older assets.
I think it's important to sit down and figure out something like a "style guide" that help all the pieces remain cohesive.
When starting a project, I'll usually make enough assets for a small enough section to see how it might go together. These are usually just enough to keep me focused on the direction I'm trying to go, but there's a chance that I might start to like the assets, in which case; I might want to keep them or use them as a style template, so I try to document the process I used to make them.
I might play around with the assets for a bit, usually when I feel like I need a break from coding or when I'm stumped on an algorithm, but I'll always try to keep notes on the process.
At some point, I'll usually find that I think the look, feel and sound somewhat come together, so I'll start trying to turn the notes that I made into general rules, hopefully figuring out a balance of time, efficiency and a quality that I'm comfortable with.
I try to keep the rules just flexible enough to still have some creative fun, but hopefully not allow for sections to feel out of place. Simple and well documented enough to allow for switching between assets and hopefully easy enough to follow that if I take a months break from working on them, I can still make something that doesn't stick out in an obvious way.
I figure everyone is different, but I always feel best when I get to do a little coding, some visual stuff and some audio. Switching between them can be a nice break and sometimes help me relax enough to figure out bits I get stuck on. I think that if I completed all the coding and just had all the visual artwork or music to do, I might get burnt out, but who knows? Maybe that's the smarter way of doing it?
I'm interested in hearing how other people deal with this.