Khyber here.
I seldom go out of my way for Conquest - usually, you get the 10% or 15% version of Conquest while taking the shortest path to quest completion. For almost every quest, I'd rather a faster completion with 15% Onslaught than adding 2 minutes to the completion to get Conquest.
I go out of my way for Conquest in those quests where I know (or can look up) the Conquest number, and know that it only involves 5-10 'out of the way' kills.
Best example: Delara's part 4 (Thrall of the Necromancer) - assuming it's being farmed on Hard, going from Onslaught to Conquest is 489 more XP (Edit: 489 that is modified by a couple of things - Heroic Inspiration, ship buffs and XP pots, so possibly as much as 1.29x489 which is a little over 600). Onslaught comes pretty naturally when doing the quest, Conquest takes 10 or so out of the way kills in the first room. Those kills add a few seconds to the completion time (usually leaving melees behind in the eight lever room while casters zerg ahead), but are well worth it.
Ransack, on the other hand, isn't worth it in that quest unless it comes naturally - 244 XP (difference between Vandal and Ransack) isn't worth running around searching for the breakables that you missed. You should still smash everything you see (and Ransack isn't hard to get in this quest), but if you miss something, no big deal, just settle for Vandal or even Mischief.
Oh and edit to state: I'm quite willing to suffer overlevel penalties for some quests. If for whatever reason I don't get around to VON3 until 11, I'll usually farm Normal and suffer the 10% penalty but do the runs faster (exception is a guild/channel group where you know people can all handle Hard fine). This goes triple when PUGging with untested players, or for quests (especially Crucible and other trapfests) that really scale up in difficulty on higher settings.
If you get lots of other bonuses naturally when doing the quest (say Conquest, Ransack, Flawless, Persistance and Ingenious Debilitation for a total of 75%), even a 25% overlevel penalty is only about one seventh of your XP.