Hello,
I wouldn't recommend doing this, since if a user consistently isn't getting ads for some reason (slow connection, in airplane mode, etc), you'll have an infinite loop of the SDK trying to request ads. That could be contributing to inflated requests and lower fill rates.
If the Android SDK can't get any response back the first time, it will by default refresh every 60s until it gets a response. At that point, your refresh rate you specified on the AdMob website will be used going forward. This means you don't have to automatically kick off your own refresh - you can wait for the SDK to do it for you at a reasonable rate.
Thanks,
Eric