And of course the major aspect that is always overlooked is that most of us do fill out our taxes to the best of our ability and honestly. Just as we may have differences of interpretation here as to whether a button replaced by a camera makes a garment a 'uniform' or not, so would just about any two CPAs you had the discussion with or an IRS agent if you had a discussion with them. They would all caution you on the side of NOT claiming it, but in a routine audit the agent might or might not agree with you. If there were significant other 'issues' with your return he would be more likely to disallow it. Recognize that it is in the best interests of a tax preparer who is going to defend you at audit for no extra fee to let you claim nothing that might even possibly be argued by IRS. And even IRS agents have specialties of situations they deal with, from what I understand, because overall the tax code is so complex and in some cases the courts have ruled directly against the rules generated by IRS from the tax code. The question to be asked is whether the 'uniform' question is something you would be ready to stand your ground on?
Another example. IRS says that a watch is NOT an allowable business expense. You cannot perform our business without a watch. I own personal watches but they don't have second hands or stop watch features. I understand that IRS does not want me buying a Rolex and charging it to my business as an expense. But I WILL deduct the $10 Walmart watch I buy about every other year with its rubber band, clear digital display and stop watch. Should I ever get audited I will wear my personal watch and throw the cheap digital in my purse for 'show and tell'. As far as I am concerned, it is 'office supplies' just as much as my clipboard and note pads are.
CPAs generally have a good overall knowledge of tax law and tax codes, but specific situations may be out of their area of expertise. When I was still in the stock brokerage/financial planning business we used to encourage our clients to bring in their CPA prepared returns before final filing. We found a discouraging number did not know how to handle wash sales or long versus short term capital gains/losses. And forget about them knowing how to calculate or handle accurately the taxable and non taxable portions of annuity payments where the annuity was purchased with after tax dollars. These days I gather the annuity companies are sending out that information, but it hasn't always been that way. IRA custodial fees when paid from post tax funds rather than by the IRA itself was also a mystery to many CPAs. It got to the point that we were sending all of our clients to their choice of 3 CPAs in town who 'got it'. And I suspect those CPAs would not do a decent job with a mystery shopping business.
My own return always has so many odd things about it that it makes sense to prepare it myself. It has investments, it has my farm, it has my mystery shopping and a few other off-the-wall things most every year. It would cost me a bloody fortune to have it professionally prepared, which is why I so love TurboTax and the surprisingly well indexed IRS.gov website.