I would recommend using one alone, because I do it all the time. Survivor bias! ;)
With SS/Stronglifts/Whatever, you begin with an empty bar, (or no bar if you can't squat at all), so the weight doesn't start out being something you need any genuine assistance with. With the power rack you have the safety pins. You adjust the safety pins to be at a point where if you are in the bottom of the squat, and you can't return to standing position, you just drop the bar off your back onto the pins (or lower it onto the pins)
With the bench press it's the same thing - you set the pins so they're below your lowest point in the bench press (bar touching sternum), but slightly above your neck level (I'm lucky in terms of bench / safety pin positioning, it's perfect for me) - when I fail a bench press, or can't push it off, I just let it hit the pins and slide out from underneath the bar.
Youtube example of what I'm talking about: http://www.youtube.com/watch?v=pCOo3RtKu1E#t=59s
If you don't have access to a power rack with safety pins I would not recommend doing the bench press without a spotter unless you back the weight off to something you will always confidently re-rack safely.
However a spotter with the squat is a recipe for ruin, best to just rely on the pins. Supposedly designed to handle 200kg+ dropped on them from 4+ft. Hopefully your gym hasn't bought a cheap shitty rack!
The thing I've come close to injuring myself the most with is the press. I was arching my back too much and putting a lot of pressure on my lower back - trick is to keep stomach tight when doing the press. Something I wasn't doing.