I weigh 120 pounds, and not infrequently dive a steel 130. But I also dive dry, so I have redundant buoyancy.
To the OP: It's good you are thinking about this. Steel tanks make a lot of sense in cold water, where we need to carry a ton of weight, and carrying more just to sink the tanks doesn't make a lot of sense. In warm water, with minimal or no exposure protection, steel tanks can present a risk. A single steel tank of normal size (and not the ultra-negative Fabers) usually won't be a big issue, because the tank starts out only 1 or 2 pounds negative, and holds no more than 7 pounds of air. If my 60 year old non-athletic body can swim up 10 pounds, anybody ought to be able to do it! Where you get into serious trouble is with thick wetsuits, or double steel tanks.
Any time you are diving with a rig that is significantly negative, you should consider having surface flotation. If you aren't diving dry, having a surface market buoy or lift bag you can inflate to hang onto on the surface is not a bad thought. You can, of course, also jettison your gear on the surface, but sometimes, having the gas in the tank might be useful, if you are in rough water.
