Watch the Playing Techniques online guitar lesson by BJ Baartmans from Creative Slide Guidebook
So as you see, there's absolutely no rules about how you approach the notes, it's just about personal taste. But, it can make for a big difference, and it's really nice if you have the difference options at hand. If it works, it works!
If you have really good control over the slide, you can even use it for traditional pull-offs and hammer-on techniques. A light touch is essential to make it sound right. You can also combine classic rock techniques with the slide taking advantage of open strings.