If someone has the courage and interest to send you an email, not only do I suggest you reply with a thanks or no thanks, I also suggest you reply soon – especially if you’re interested.
Some online daters take days to reply to someone they’ve actually showed an interest in. It’s as though they sent out numerous emails to to a number of potential online suitors then waited for replies. Then, they check out the “better” ones first before getting to the bottom of the barrel.
Think of how insulting that is. I guarantee that anyone with any self-esteem will stop writing to you if you make a pattern of not showing interest fast enough.
Besides, it comes down to courtesy. If someone writes to you, reply in kind. How would you feel if you were treated like someone at the end of a line?