I've always glanced at the 5 year holding periods and found they didn't apply, and so ignored them - can't help if those apply to you.
To be clear, when you sell to cover an RSU, you get shares. So you're no longer dealing with RSUs, but with shares held in a brokerage account. Of those shares, selling those at a loss makes the most sense. The IRS "shares the loss" with you. So if you lose $300, in the 33% bracket the IRS will absorb $100 of the loss.
Next up would be picking from those shares held 366 days or more. Pick the lowest gain that also fits in the long-term capital gains rate. You save by picking a smaller gain, and by limiting to the better tax rate.
Note you don't have to "sell to cover". You can just sell, and use the profit to invest elsewhere, like in an S&P 500 fund. That way your profit grows diversified, instead of in company stock.