It really depends on how you want to spend your time leveling. If you see yourself fighting one mob until its dead and then moving on to the next mob then spec destruction from early on. The points that reduce your shadowbolt cast time and shadowburn will help you to almost not even need dots. However, if you see yourself fighting more than one mob at a time then you should consider getting instant corruption early on. Being able to tab from mob to mob and lay down instant corruption and curse of agony will kill everything without shadowbolts...

Also, the best leveling spec for a warlock, especially solo, imo is affliction. Leveling isn't about survivablity as much as it is about killing quickly with minimum downtime. Affliction is the king of efficency. Demonology improves as a leveling spec 50+ once you can get felguard.