I don't think you are required
to carry over losses. But Topic 409 is pretty clear that if you choose to use carried over losses to offset gains, they are required to be losses carried over from the previous
year. The loss carried from this year to next year is calculated based on this year's return. If you do not apply last year's loss carry over to this year's return, then you will not be able to carry it to next year, and the loss will be... uh... lost, pun intended.
But I am not a tax professional, so under no circumstances should anyone consider this tax advice.
Tax Topics - Topic 409 Capital Gains and Losses
"Capital gains and deductible capital losses are reported on Form 1040, Schedule D (PDF). If you have a net capital gain, that gain may be taxed at a lower tax rate than your ordinary income tax rates. The term "net capital gain" means the amount by which your net long-term capital gain for the year is more than the sum of your net short-term capital loss and any long-term capital loss carried over from the previous year
. Generally, net capital gain is taxed at rates no higher than 15%. However, for the years 2008 through 2012, some or all net capital gain may be taxed at 0%, if it would otherwise be taxed at lower rates."