Invariably, I ease the topping lift once the main is up. The side the line lies is simply determined by whcih side the roach is lying as you go to windward to hoist the sail. It pays to always watch the leach/roach as the main goes up as it can snag on the topping lift with part of the sail lying to the windward side and part to the leeward, in which case you ease the line early to avoid damage to the leach and any battons affected. Once the sail is set, simply pull the line tight by hand only and then you avoid it both flopping about, getting tangled on things if loose or distorting the sail shape (specifically the roach) if too tight. as you tack, you may need to check the tension again to ensure it is not interferring with sail shape. In essence, it is should be lazy bearing minimal weight as you sail, unless you are using it in certain conditions to shape the sail up high as in light wind
conditions. Remember to tension it as you lower the sail so as to ensure the boom sits where you intend, and not as can happen, sitting on your bimini
frame for instance.