Paying taxes is a total pain as a small business owner. A HUGE chunk of our overhead goes straight to the IRS. Many companies fudge the numbers, avoid paying, or declare bankruptcy. A builder who doesn't pay taxes on his income can "offer" you a much better deal than we can to redo your space, no doubt.
(Excuse us while we roll our eyes)
Many people believe it’s not their problem if they are hiring a company who evades taxes or hires illegal immigrants. But if a company is willing to dishonestly handle their own business in this way, we would have some major concerns about how they are handling you as a client. Time and again we see building practices mirror business practices. That is, if your builder cuts corners in his business (like paying taxes) (which is not your problem), we can almost guarantee he will cut corners in his building as well (like in your bathroom), (which is most definitely your problem). In the long run, these shortcuts are so, so much more expensive and can even cross into dangerous territory. I hate to be cliche but it is so dang true: you always get what you pay for.
The market is saturated with builders marketing their "integrity." An easy way to ask how that plays out in business or in building is to ask a contractor how paying taxes has impacted his growth as a company. If the answer is anything besides "it is really difficult" I would proceed with extreme caution. More than that, I would run the other way.