That is indeed true, but the part I left out for brevity is the part about client expectations, which must also be handled. I disagree however that it makes you appear needy. If you ARE needy, you're going to seem needy no matter what you do. I can assure you though the client would rather have you reply sooner than later... just set the expectation that you can drop them at any time (subtly obviously, you wouldn't say that to their face).
Consider the following scenario:
A) Email exchange which takes 2 days, for a fix to a site... you quote $800 and they hesitate
B) An email exchange where you take no more than 2 minutes to reply, even at 11pm. They are happy that you got back to them so quickly, and you quote $2000. They agree.
This happens quite a lot in my experience. To the point where it's extremely noticeable and can't be chalked up to coincidence.
I just mentioned what works. If your goal is to get a huge amount of work for $100-200 an hour, then replying right away will get you significantly there (skill aside). Everyone waits hours to respond, so it's the norm. If you want to stand out, you need to be outside the norm.
As far as being at the mercy of your email, in practice this genuinely doesn't really mean any difference than if I wait. You're still going to write the email, it's just a matter of when. My productivity remains the same. If I'm very deep in a project, chances are I need a break anyway (I'm bad with taking breaks when I need them). If I'm really busy with some other task common sense would be required, obviously there are exceptions.
Sidenote: I don't necessarily do this with well-established clients who I've worked with for years. They already know the scoop. I'm more talking about new people, new leads, new projects where they don't recognize your value yet.