I was asked to rate/review, but I already did so. Will closing the dialog have any negative effect?

The Text Fiction version available via Google Play will always ask you to rate/review it after a reasonable amount of usages. Whether you rate/review or not is entirely your choice and has absolutely no impact on the app itself. Your options in detail:

Rate

When you choose to rate, Text Fiction will open the market client with the right page for your convenience and never ask you to rate again afterwards.

Don't rate

Text Fiction will never ask you to rate/review again.

Postpone

The usage counter will reset and you will be asked again at a later time.

No matter which option you chose, your choice is only stored locally on your device and not used for anything but the reminder dialog.

Please note: while your choice has no impact on the app itself, it has impact on where development is going:

  • Rating/Reviewing helps Google to better place apps, which in turn helps growing the installbase. An active userbase is what keeps open source projects alive.
  • A good review/rating is like a pat on the back for developers and encourages spending more time on the project.
  • Giving feedback often provides ideas for new features or improving existing ones (feature requests/bug reports should are better sent via e-mail, though).