I'd advise against asking "on initial startup." General consensus seems to be that users hate being asked to rate apps with a pop-up (regardless of the content), and developers risk negative ratings for asking too early, too often, or too intrusively.
This is especially true:
- Right after start up, when the user wants to accomplish something
- When it randomly interrupts their action based on an arbitrary timer (5 uses, 10 days, etc)
- When you ask within a very short time of first installing the app, before they've even had a chance to use it & form an opinion.
Best practices & options are still a matter of debate, but in general have started to steer away from this approach.
Some good suggestions to avoid berating users include placing your rating/feedback area in an "about" view with your contact info. This has the added benefit of providing a users with a clear alternative way to report bugs/crashes - hopefully they send you a private email instead of leaving a 1-star rating. If you just say "Rate Now?" you get what you ask for...
You could also opt for a less invasive way to ask than an alert (like a banner-ad style subview). This Tumblr has some good & bad examples of things you could consider.
Sending emails to (willing) subscribers, or including a reminder in the App Store copy to leave reviews after each update could also help.
"But Flappy Bird got ranked #1 with its big ugly alert! Why can't I?"
True ... placing the "Rate Now" pop-up directly at the time/location of the "Play Again?" button is one reason cited for Flappy Bird climbing the charts from obscurity to fame and fortune.
The key difference though is that the pop-up appeared strategically, after the user had played a few times, and right after they'd died, a clear stopping point/milestone. It wasn't the same interruption as a random alert getting in the way of the task.
The placement over the "Play Again" button sort of tricked people into leaving a review. This surely angered/confused some people & its success isn't likely to be repeated, but the sheer volume of ratings had the desired effect, and eventually it took off.
If you do go with the pop-up alertview, then the 3-button option with "Never Ask Again" is safest. If people like the app & want to leave a review, they will.