You usually dont use only the spring for ledge gaurding, its better to walk/spin/asc off into an aerial, (preferably bair of fair obviously) and then you use up-b, not only to recover but also to drop a spring. basicly what it comes down to is the spin, the aerial, and the spring will all have a chance of hitting the opponent, and possibly even stage spiking. that is 3 easy chances for kills, or knock aways.. .
Spring works best if you use it when an opponent is right under sonic, tends to stage spike more that way.
you can use HA also, but its more risky imo. I have killed people with it a few times tho.
The spring can be used in a lot of situations, I have found that its definately not the spring that kills you, its the coming down. Like tenki says, dont just dair after every up-b.
BUT its important to note that it defeinately is ok to dair after up-b, usually you just have to alter the timing of it a bit. So then people expect immidiate dair, they do their punish on their standard timing, you can dair after that, or you can come back to the stage with a bair. even a ASC or HA if they dont expect it.