Sally??? Is that you? We miss you over in Dirt!
For me, I start the season with my best players in there based on ratings. If I have a hefty platoon split, I usually stick with it all season and use both players in the platoon as a player rest at a separate position. I usually don't mess with any lineup too much until I see 250 at bats. If I have a guy having an up year and a guy having an down year, I'll adjust accordingly somewhere between the draft and the all star break. The splits stats (vs. Split ratings) for batting vs. LHP is small sample size nearly all season so I stick to their overall batting.