I find, personally, that Edge is most useful at 4 and up. At 4 or 5, adding dice (and thus the Rule of Six) is really worth it, because even tests you default on will have decent pools. At that level, Burning Edge is more possible for the truly screwy moments. I generally recommend having 1, or having 4 or 5. Doesn't seem worth it to me at 2 or 3. If you want to use Edge, generally go for high. Otherwise, low. You can always buy it up later. I think hardcapping is rarely worth it without going fullblown Mr. Lucky, or with long term runners running out of things to spend Karma on.
I may be slightly biased by my preference for karmagen, though, which makes the min max less an issue.