A customer driven email template/color theme should bump up your open rates too, especially since your visually personalized email has a higher chance of recognition during a preview pane scan of emails.
What if Mr. Smith your competitor, goes to your opt in page and signs up as Mr. Idiot by entering one/some of your potential prospects email addresses and your automated mailer program send's out a highly personalized unsolicited email addressing them as 'Dear Mr. Idiot'?
Welcome to the murky world of 'List Poisoning'. Not only do you get flagged as a spammer, but your company would have earned a set of die hard enemies, who'd do anything and everything to tell everyone who the real idiot is.
Another ploy of list poisoners is to optin using mulitple spam trap email addresses to get you flagged as a spammer. Often the optin's originate from the same ip address in quick succession.
Preventing List Poisoning: Unfortunately you might not be able to prevent someone entering incorrect data in your opt in page, but you could always quarantine the opt-in list, review/validate it for data accuracy, and then send out a confirmation email before actually adding them to your mail list.
This would affect your opt in rate adversely, but sacrificing on a few not too eager prospects might be worth it if you can prevent making a set of seriously annoyed prospects. It's all about choice.
So you'd have to add another segment criteria based on 'preferred send time' to ensure your emails reach your customers at their preferred time.