I was investigating Income Based Repayment the other day, and eligibility depends on AGI. I've been making Roth contributions in the past, but hadn't considered that AGI might gatekeep government and non-profit benefits. Beyond reducing income tax, what benefits accrue lowering your AGI? Conversely, are there costs I'm missing?
I look at it this way. You are trying to decide between a Roth vs traditional IRA, or 401(k). Using tax software you can see a number of scenarios pretty easily. Look at the difference in your tax bill if you flip the last $1000 from a pretax to post tax deposit. This will show you pretty clearly the effect of whatever phaseout of benefits you'd have based on this change of AGI. By tinkering a bit, you'll see exactly how and where that phaseout impacts your return.