So you reinvest dividends, STG, LTGs in taxable accts and pay the taxes, but when you sell you don't pay taxes again on that amount. Who keeps track of that and do you specify when selling later?
The mutual fund company, tax guy, me and tax guy??
I disagree with RunningBum's reply.
You, the taxpayer, are *always* responsible for proper reporting of cost basis. It's your signature at the bottom of your 1040 saying things are accurate, not your mutual fund company or your tax guy.
There was a change a few years ago that mutual fund companies are required to track and report basis to the IRS for certain investments purchased after certain dates starting in 2011.
However, for shares purchased before those dates, basis information may or may not be reported to the IRS. Further, the basis they have could be wrong. The basis might be missing. Or it may not match the cost basis method you've elected to use. It also might not take into account transaction costs. Finally, it won't take into account adjustments to basis, such as QSBS.
You can choose to rely on your mutual fund company's information, and it's probably correct most of the time, especially if you haven't done anything complicated and the mutual fund company is one of the reputable / capable ones, and especially if you bought covered shares.
Here's a Vanguard web page that talks about all of this:
https://investor.vanguard.com/taxes/cost-basis/covered-noncovered
I think your tax guy is only responsible for knowing the tax laws, making sure you're giving him all of the relevant information, and accurately and competently completing your tax forms.