Swellow-Soul - It really depends. I'd say the variables are catch rate, difficulty of area, experience, and how you're feeling (lol). The higher the catch rate, the less conservative you need to be. The harder the area (Sandstorm, Trophy Garden, etc.) the less conservative you want to be early in the chain because your chains are likely to be broken more frequently, so being conservative the whole time could result in a long time spent on relatively short chains. The more experienced you are the more conservative you should be because you have the ability to be conservative successfully, whereas rookies might not know exactly how to be conservative well. If you're feeling impatient, I'd be less conservative at the beginning, just because =/ .
I personally just say fuck it for the first 10 or so. Typically most "decent" squares won't fail you, and if they do, so be it. But after 10 or 15, if you're careless, it really costs you time. There's no real reason to not be somewhat conservative after that time. One example is Beldum, I was completely loose for the first 20 or so, because they have an incredibly high catch rate, and a very difficult area. After that, though, i was as conservative as possible pretty much. I actually started reseting for shinies at 38, because I was terrified of breaking the chain. I actually entered my "perfect patch" at 39 though, because I was getting a little impatient.
Maybe i overdid this explanation a little, but those are the factors I base it on.
Corbin is exactly right btw. To that list I'd add that in the Sandstorm you want to do it in the morning (in terms of game time). In the snow I don't recall there being too big of a difference, and I've had success in both night and day IIRC. In the rain I can almost guarantee the morning is better, although I've never tried.