Spamming lacerate means it doesn't tick anymore. Assuming you're casting lacerate faster than once every 2.5 seconds, and knowing that lacerate ticks every 2.5 seconds, if you spam it, it will seldom ever tick, realistically only when you take some time to mangle and faerie fire.
At my low gear level, lacerate ticks for 675 at 5 stacks. The upfront damage, however, is 67 (it is not affected by the stacks). It is an insane, massive loss to spam lacerate instead of letting it tick.
Maybe on Warmane refreshing a dot doesn't reset its tick timer ? I can't test it there. However, if it works like I think it does, there isn't any theorycrafting, calculations, forum war-waging or hair-pulling to do on this matter : fill with swipe, let lacerate tick, refresh when you think it's safest, taking into account your hit/expertise and cooldown of your other abilities.
Note that even at my low, naxx-entry level gear, swipe deals around 700 damage, and lacerate 67. There may be some sort of coefficient bug where I'm playing (not Warmane), but if there isn't, it means that even at low gear, swipe will do more threat than lacerate's upfront damage, if your formulae are to be trusted. 3*700 = 2100. 1*100 + 1000 = 1100. 2100 > 1100 swipe superior even at the lowest conceivable gear levels.
Note that now that I think about it, my lacerate damage really seems to be low. I'm at 4.6k ap. The tooltip says 92 to 93 damage and yet it does 67 (it is bleed damage, so armor shouldn't matter). Maybe it's bugged on my server ? It would help if someone could test on Warmane to settle this once and for all.
But low lacerate upfront damage or not, the tick resetting issue makes it clear that swipe as a filler is the only, very obvious, option.