I didn't have to hire anyone to get the penalties waived; I just submitted a letter explaining my original rationale, along with an amended return fixing the problem and a check for the correct tax. The IRS was completely reasonable about it.

Edit to add: But then again, I do my own plumbing repairs. Heinlein didn't list taxes in one of my favorite quotes from him, but he should have: "A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects."

For me so far the problems have been easily sorted out except once. The once was a lot of extra and unnecessary calculations but essentially no penalty. Just the cost of my time, which is bad enough.

My main complaint (which is still fairly minor) is that they still too often presume that "it" (the reason why they want more money) is the fault of the individual taxpayer. No need for them to consider other possibilities. Once, a client provided poor numbers on their 1099 and I used my version. Both versions based on my invoices. IRS doesn't bother to ask for a clarification. Just sent me a bill and fine. And I suspect in the end after my response letter, they audited the client. Not the best result but only a little stressful on my side.