- Is the ACA subsidy valuable enough that it should give me pause when considering a sale that would incur a sizable LTCG?
Should it give you pause? Yes, absolutely. The premium subsidies phase out pretty gradually until you hit the cliff at 400% of the poverty level. As long as you stay under that threshold I wouldn't worry too much about a few extra dollars of gains here or there, where the premium subsidies are concerned.
There's another pretty significant cliff at 200% of the poverty level for cost-sharing subsidies (and smaller cliffs at 150% and 250%). Take a look at
this Kaiser Permanente brochure for Washington (pages 9-11) to see an example of how much of a difference these cost-sharing subsidies can make. The "VisitsPlus Silver HD" plan, as offered to people over 250% of the poverty level, has a $7,150 deductible for most services, but primary care visits are a flat $30 co-pay and specialty office visits are $55. If your income is between 200%-250% of the poverty level, you get the "VisitsPlus Silver 73 HD," bringing these numbers down a bit ($6,250, $20, and $45). Below 200% is where you get really subsidized ($1,900, $10, and $20). Go below 150% and you get subsidized a little bit more ($775, $5, and $10), but I see 200% as the real cliff to consider.
Now,
@Financial.Velociraptor's point about not letting the tax tail wag the dog is 100% valid. Don't make irrational choices just to get your taxes down. Selling a bunch of stock one year may well be in your best interest and the ACA hit is just the price you pay to make that worthwhile transaction. Do be aware of the tax thresholds involved and take that into account when making your decisions.
- Given that my expenses are covered 100% by dividends only, should I not even worry about losing 0.1% of my stash to LTCG taxes?
If you don't need to sell any shares to pay your bills, I wouldn't be in any hurry to sell shares. Even though the nominal federal tax rate is 0% for low-value capital gains, the ACA subsidies and state taxes (if you have those) will mean you do pay something in taxes for even selling a few shares. Deferring tax liability is generally the best course of action unless you have some real compelling reason to sell.