Agreed.
In order to become especially proficient at needle camping, its important to consider what needles themselves are actually doing. Look at the following picture:
URL in case the image doesn't insert:
http://imgur.com/gallery/jIg79Km
For the slight amount of time that needles are active, that amount of the stage 'belongs' to you. You're limiting the stage options the opponent has, as they cannot traverse the majority of the stage as they'd like. They either have to jump to avoid the needles or shield them (which means no mobility). Since needles are so effective at this stage control, as well as their general superiority to all other proectiles, they effectively act as a magnet on your opponent. You can just sit there and throw needles at them, taking control of the stage while simultaneously damaging them, so they have to come to you.
Different opponents will have different measures for tackling our projectile. They can jump over them, do run > Shield > run, or their character might have a specific attribute well suited to punishing stationary projectiles (Bouncing Fish, for example). I actually find that needles in neutral is a great way for getting a feel for the skill of the other player. Better opponent will have much tighter methods of getting around it, while newbies and scrubs will usually deal with them in ways that play right into our hand, which brings me to my next point.
Since Needles are almost the ultimate neutral option (at least in terms of projectiles), the opponent has to adapt and choose options that counter-act what we are already doing off the bat. Scenario: We throw needles, so the opponent approaches with running shield as their primary counter-option. Here we already have them within our hand, as since our needles prompt them to shield, we can run up grab them in neutral due to them
expecting needles.
This is why the idea of 'needle camping' isn't so straightforward in my opinion. You shouldn't approach needle camping as both the means and the end. See it as a low-risk method of leading to greater punishes. Needle camp at points, but use it gain information on their preferred neutral defenses and then move to punish those chosen options with the appropriate method. To this extent, I'd include both F-air and Needles within the 'definition' of needle camping (instead of just needles). Needles for the initial, long range zoning, but F-air for the close range encounters as your opponent makes it closer to you.
As for when, I'd say that it depends on the opponent (just like most things in this game). As a general rule of thumb though, needle camping at low percents is good as an approach-inducing option (relates a lot to all the stuff I discussed above). At high percents the disruption factor of needles grows as the opponent is flung much further by them. This is great for frustrating the opponent into choosing a dumb option, as well as just keeping them away. I wouldn't doubt a Sheik's ability to time out a Ganon by just needling and spacing F-airs well.
I'll leave it there for now since its 6AM over here. If I wake up, read this again and throw up a little in my mouth, I'll probably redo it.
