Alot of questiuons like this can be answered by asking yourself a question....
"How much time and how much money will it take to repair the part compared to how much is it for a replacement part?"
If it will take alot of time to fix it and still be questionable, then why not replace it? Especially in the case like this rear bumper. You will spend hours trying to fix it and get it repaired, or you can spend about $50 to get a replacement. Now, if it was a dash that cost $800 for a NOS one or $525 to have it professionally restored, then you time to fix a dash might be worth it. So, weigh the cost between your time and money to fix it, or a simple money to replace.