The IRS imposes an underpayment penalty if you owe more than $1,000 at the end of the year unless you qualify for one of the exceptions. The most common exception is if you paid more than
Your broker will not withhold taxes from capital gains. If you anticipate this happening in future years, you can either file a new W-4 with your employer asking them to withhold additional taxes or pay quarterly estimated taxes directly to the IRS.
Most people who do receive a W-2 find it easier just to have their employer do it. That way they don't have to remember every quarter and then you don't have to remember to report them at tax time. The IRS will not send you a tax document for estimated taxes that they have received throughout the year, whereas the additional taxes your employer withheld will be reported on your W-2.
