I'm sorry, I just re-read your post, and you said 7-speed cassette. I was thinking 7-speed freewheel, probably because I have (until recently) been running a 7-speed freewheel with a single-chainring crank on my commuter bike. Freewheel hubs did commonly suffer from bent axles, especially with bigger riders and especially with 7-speed freewheels. Not so much a problem for 5-speed freewheels, as there's less axle sticking out between bearings and dropout.
If you're going to get a cassette-hubbed rear wheel, you may as well get a standard (modern) one that is compatible with 8/9/10 speed cassettes.
The only reason to go with a 7-speed freehub: if your frame has narrow spacing (124 or 126mm rear dropouts) and you don't want to "spread" it a bit wider to fit the 130mm-spaced modern road freehub in between the dropouts. With steel frames, spreading a frame 2-3mm on each side isn't any problem.