Running straight pipes does not harm the engine. Running open headers does not hurt the engine either.
Thinking years ago was that cold air hitting the exhaust valves could damage or warp them, but when A.I.R. pumps came into use to curb emissions, that "thinking" went away too.
Straight pipes may be a little loud and a few neighbors (like me!) might be a little hostile if you run them, but the choice is yours.