The IRS does care and will use the amount reported for Form W-2G for your gambling winnings.
Per Shollenberger v. Commissioner, T.C. Memo. 2009-306 (2009) you can report net winnings on a per session basis. A session would be when you started and when you stopped. The IRS may audit you, in which case you will have to produce detailed records.
Under Shollenberger, gain and loss is computed as the difference between what you started and ended with. However, you can only net down to $0. Anything beyond that would have claimed separately as a gambling loss.
If you started with $100 and left with $200 you would have a $100 gain. If you started with $100 and left with minus $100, you would have net gambling income of $0 and a separate loss of -$100. You would report net gambling winnings of $0 for that session. The loss goes elsewhere, i.e. you can't have negative net gambling winnings.
As for what a session is, in LaPlante v. Commissioner, T.C. Memo. 2009-226, the court noted, "Respondent [the IRS] nonetheless agrees with petitioner's theory of recognizing slot machine play on the basis of net wins or losses per visit to the casino."
