Big, burly guys would be a sign of relative civilization, and nomads/hermits would be smaller and leaner.
If you are fending for yourself, or are a part of a very small group of people, you will probably benefit more from endurance (to allow you to follow potential prey over a long distance) and speed (for outrunning predators/enemies) rather than strength, assuming you have some sort of decent baseline strength to begin with.
Being strong is super useful, but you'll be bigger and heavier and use up a lot more calories, which means you'll need to hunt/forage more. This not only increases the amount work you need to perform (and thus ups your calorie requirement even more), but it also increases the amount of risks you have to take - you may run into other people looking to harvest the same resources you're out after, and as @Kilisi pointed out, being muscular and strong is not much good against five people tossing javelins (or shooting bullets) in the general direction of your face. Or, in the case of hunting, the resource itself may oppose to your gathering it, and bite you, kick you, or shove a horn into your soft parts. Or you could just take a wrong step and sprain an ankle, rendering you unable to walk for three days. If you're lighter and need to do less work, your calorie requirement could easily be half that of your juiced-up counterpart, so you can spend a lot more time just chilling.
However, if you are part of a group large enough to justify specialization between individuals (e.g., a village where people have different occupations), the cost of making one or two individuals abnormally strong might be worth it for the group (especially if technology and/or animals offsetting the need for strength are lacking). These guys would have an occupation requiring good strength, like blacksmith or stonemason, but would also serve as a sort of communal resource anyone can call in when strength is needed.
As for negative side effects, there are plenty, of course, but they vary a lot from person to person. Some people die from live failure on their first cycle, others are mostly fine after doing lots of steroids for many years. Like a box of chocolates, you never know what you're gonna get. Also, there are different steroids, which have different side effects (some are more bound to make you aggressive, others are more dangerous to your liver).
What does all this mean to your game? Well, perhaps steroids are easier to find in a walled village rather than in some forest-dweller's shack.
If you want to justify the abundance of steroids in your game world, perhaps the pre-apocalyptic civilization were really into steroids, and managed to elaborate safer formulas with less Very Bad side effects (like instant death) while still having some Fairly Unpleasant side effects (aggressiveness, long term health problems). Seems like a interesting contributing factor to the civilization's downfall, too.