I like your thinking - however keep in mind that the learning curve may be much steeper and quote availability for backtesting is very limited.Why not learn where it's hardest, then liquid is easy. I don't mean that in as a way of torturing yourself, but rather that there are fewer trades, less motion, etc, so if you can get your algorithm to trade that then it's a great exercise.
Let's say you find an algo that on average loses 20 cents per trade. On top of that you have 2 cents for the spread plus commissions. You reverse the algo and all of a sudden you've a winning strategy that makes 18 cents minus commissions.
Now you take that approach into after hours land where the spread is 50 cents or higher, you'll end up with two losing strategies.
But back to your point - it may hone your skills enough so you can actually find more profitable setups. Start out there if you want and if there's little success you can always move it to regular trading hours - that's the beauty of algo trading - Good luck and let us know how it goes