Increase your rates regardless. Clients don't know, or really care, about quality so long as what you build works.

Where quality is important is in keeping your costs low. Having to revisit old code to fix bugs or trying to work with bad code to add features has very little impact on your client. It affects you. Strive to write good code for your own sake.

Plus, if you take on fixed price projects writing code you don't need to go back to really increases your profit.