I think that may be an old-wives-tale...my optometrist told me I had candle-itis (too many birthday candles). He told me he could sell me some readers for $100, or I could go to the drug store and by 3 for $10 and have 2 spares.
He did say to try the exact pair you're thinking of buying.
He said that if I bought his, they'd be 1.0x +/- 0.05x on each lens, but that some of the drug store pair might be 1.0x +/- 0.2x on each lens (so you could have 0.8x L / 1.2x R).
As to the bifocal safety glasses - they're cool, but...the magnifying part is pretty small, and it's sometimes hard to get your head tipped just right to get in the magnifying part.
I keep both bifocal safety glasses and regular safety glasses and readers in the garage.