Hi,
Monitoring the memory is important to track memory leaks or to anticipate memory upgrade if you know the router is going to handle more and more routes for example.
Same thing for the CPU. It may not be normal a router start having CPU peaks.
So I think you should check what is today the average of CPU and add 20% to this value to have your threshold for investigation.
Regarding the memory, it could depends of how fast your network is growing for example so you have time to anticipate any memory upgrade...
HTH
Laurent.