The problem I tend to encounter is that even if I manage to force him to back off early-game, he still drops poison on the mobs, which means that I can't effectively sustain with spellvamp/lifesteal early on. Maybe it's just that particular harass style, but I've yet to encounter a Singed who does anything but run in, drop poison, maybe flip me further into it, and then back off again. If I chase I will drop too low and die, or not catch him because I'm avoiding the trail. If I stay I lose health and by the time I regain what I've lost he's already come back and harassed again. If I back off I still have to deal with poison, and he takes over the farm. Basically, I don't have much of a problem ganking a Singed under other circumstances, but I really can't deal with his lane harass, especially when the player basically never comes into melee range, so I have a choice between taking a little damage for no return or chasing and dying.
Basically I can keep him off while I farm early on, but I take damage for no return unless their player is really daft, and once he hits level six trying to nail him when he can see it coming is at best a zero-sum-game, which is usually when I start roaming. God forbid they send a second player top, then I either call for help or feed. I probably need to work on my Q+E more to get a better feel for the outer range it works at, so I can intercept more effectively.