A post-interview follow up is important to candidates. They took the time to come in and interview, so will look favorably on your company if you follow up to tell them what they should expect next.
At the end of the interview, have an exit conversation with each candidate thanking them for their time and giving them some idea of when they should expect to hear back from you with a decision. If you give candidates a definite answer on this (for example, "by the end of the week"), either be sure to actually follow through in that time frame or send a follow-up email to let them know about significant delays.
Note
Make a note
Why spend the time and energy following up? Every candidate has experienced the company who tells them that they'll hear back within a certain number of days and then never calls, or the company who goes completely silent after the interview and calls back months later with an offer. You may have very real organizational issues that cause this...