If you do not file a tax return for a given tax year, the statute of limitations for that year never runs out. Therefore it is advisable to file a return for each year, even if your taxable income falls below the minimum amount required. This is done in order to cause the statute of limitations to run out.
If you do file your tax return each year while living abroad, the statute of limitations for IRS audits will expire three years after you file those returns. This means that the IRS cannot go back and try to audit or challenge those returns later. Therefore, you should file your return even if you have no income or do not owe taxes in order to force the statute of limitations to run out and eliminate future problems when you decide to return to the U.S.