My guess is that the second option is your best bet. That's how I had my small layout setup. Back then I found a website that suggested where to locate turntables (part 3680) to specific places in curves to secure the track. Running your train slowly should not pose a derailment risk if you don't support the curves. But if you've got the plates, by all means use them.
Also, you don't have to support the entire sleeper. When running out of 2x8 plates, even 2x2 plates on the edges of the sleeper, under the track is enough support.
-Juan