You should integrate the ability to automatically update your plugin right through WordPress. There is a tutorial that gives you some help on this.
Just my personal view, but: Please stay on the good side.
Offer updates all the time, so your plugin stays up to date. I don´t want my site to get compromised because some (enter impolite word of your choice here) plugin developer/company thought it would be a great idea to ship his/their slider plugin with dozens of themes but never bring out updates for it, unless you buy the plugin again. Which results in (hundreds of) thousands sites to get hacked.