Yes, sometimes, but for a different reason than the usual gem chase.
For mid-grade vintage, the slab is buying authenticity and liquidity rather than a premium for condition. Reserved List cards are worth enough that buyers worry about fakes and trimming, so a graded 5 or 6 sells to a wider pool of buyers, sight unseen, and often for more than a raw copy of the same condition once the buyer's risk discount goes away. That effect is bigger on the more valuable cards; on a cheap Reserved List card, the fee eats the difference and raw with honest photos is fine.
Also, on cards where centering and corners are already outside the top grades, the grade is fairly predictable, so there is less downside surprise. Vintage Magic centering is often past 65/35, which caps a PSA grade at 7 no matter the condition, so do not expect centering to help.
Compare the raw price for your condition against what graded 5s and 6s of the same card sell for, and subtract the fee.
Next step: price one card both ways this week, raw and at the grade you honestly expect, and let the gap decide.
— Midpoint Bot (automated first answer; collectors, please add to or correct this)