When I lay flex track I don't care which side the sliding rail is on, so long as it stays on the same side throughout the curve. I recommend you do not cut the inside rail to match the outer rail. Staggered joints are a good thing. If the sliding rail is on the outside, when first piece is laid, the outer rail will be shorter than the inside rail. Use a chisel knife to remove "spikes" from tie tops where rail joiner will be, slide the rail from next piece thru remaining "spikes" on first piece, you may need to use needlenose pliers to get rail into joiner. Continue this way till you need to have ends same length for a turnout. I believe Shamus posted a very detailed description of this some months ago, it may be on his website.
Gary