If you use the bookkeeping programs that most computers come with they will do your taxes for you, the good ones at least - I use Quickbooks. They keep a running tally of the items, come the end of the year, you just print and fill in the blanks on the yearly update that keeps track of the changes in the tax code. It works for me, and I get a pile of W-2, and have three income streams taxed at two different rates.
In the end, you and only you are responsible for them, if H&R or just about anyone else, messes up, it you, not them that pays the fines. (But most of their mistakes benefit the government, which is even worse I think) The only exception, and only people running a large business ($100K and up per year) need one, is a lawyer that is a CPA. That way they can defend in court for you.
You can also use the programs to keep track if you are paying too much or too little and adjust it as you go, no big shocks at the end.