When we travel to major cities, we generally don't rent a car if we can avoid it. Part of this is obviously the cost associated with a rental (daily fee + gas + parking can add up for sometimes minimal use), but also because if a robust public transit system exists, we'd rather just use that. We love a good metro even if it takes a bit longer to reach our destination than driving.
Anyone who lives in Pittsburgh knows where we're going with this because our public transit, well, leaves a lot to be desired- this statement is being generous.
If money was no concern, we'd state outright that you'll do well to rent a car in Pittsburgh- you'll have way more freedom than if you didn't (and also have access to a number of great day trips too). If money is a concern, you can make it work without a car but you need to be a bit more strategic with how you plan your stay.