WSDOT has a lot of good informaton, maps, times and cameras. Google Maps also handles the area well. You can choose a future travel time; future times are a range, since exact conditions will vary. But it highlights common tight spots.
I5 won't have the heavy downtown oriented traffic (unless there's a game), but Saturday thru traffic can still be heavy. 405 isn't much different. There is a freeway alterternative using 167 that bypasses the core Tacoma traffic, though it still hits a tight spot around Renton.
For minimal traffic, you could take 101 north to Port Townsend, and then the ferry. That ferry is short, but reservations are strongly recommended.
