Morning or mid-afternoon are best. If you believe a decision is imminent, then send an Email thank you letter and follow it up with a handwritten note. The goal is to thank the interviewer for his time and reiterate your interest in the position. A personalized letter or email will make the best impression.
This is especially important for an interview thank-you letter; you want to remind the interviewer that you are a strong candidate before he or she makes a decision.
If you plan to follow up, you can let them know here when and how you will do so. Thank-you notes aren't just for job searching.
For instance, if you are an IT professional, an email letter is most welcome. Describe the present a little, so that the reader will know that you have taken your time in actually cherishing what they gave you.