I have to agree with the conditions numbnutz stated and also add that it depends on your setup. If you have scouted your setup well you should know when you need to be there. If you haven't then I would wait for shooting light to start.
Lots of really good points and info in this thread.
In our case, weather plays a big role, but we mostly base our tactics on the other hunters. We are on private ranchland that backs to pretty heavily hunted State trust and NF land. However, we're @ the bottom of the drainage (and puiblic camps are roughly on top), so that makes a big difference in our tactics.
Thus, in most of the rifle seasons, the elk move (largely downhill) based on the hunting pressure, so we use the crowd to our advantage. Generally - opening morning or 2, the public area camps will be up and at 'em and in the woods on top of the drainages well before dawn...so those first two days, we'll be set up in some honey holes down near the bottom well before first light. We move less and glass and listen more.
As the season moves on, people tend to sleep in (or go home), so the animals are moving less. So - from day 3 or so on, we're higher in the timber and stillhunting the nasty little holes.
Probably goes without being said, but in a lot of OTC area situations, using other hunters movements to your advantage is key, IME. And remember that most other hunters aren't going to move too far from camp. In our area (and I'm sure this is true a lot of places), there's basically a demarcation line fanning out maybe 2-3 miles from the groups of camps. Beyond this line is where the animals will be.
Use Google Earth and look @ where the highest concentration of camps are in your area and draw the line. We're pretty intimate with our ground (having the land in my bro-in-laws family for a century helps that), but I've found a couple of great 'new' holes/escape routes the last two years just from using Google Earth and overlaying common routes of attack that the other hunters use and what the routes of the animals are likely to be in response. My nephew setup on one of these holes last year and killed the nicest bull we've seen there in a decade about an hour after daybreak (i.e. just long enough for that bull to have bolted a couple of miles downhill from the pressure above).
Man - I'm excited for elk season...