The system logs are automatically rotated and/or deleted when your mac does it's housecleaning (assuming you leave it on). If you dont leave it on all night, use one of the many maintenance utils available to do it manually.

As far as install logs, do what you want with them. Only reason to keep them is if you want to uninstall something and want to make sure you get all of it.

Crash logs can be vary helpfull when determining "What the heck happened?".

Personally I recommend Applejack to do cleaning, but there are many others just as good.