I try to do it as early as they post, but usually all we've been getting is people who come on here for advice or help and dont post any replies to what other members have to share with them for assistance. Now, I only welcome those who have at least 10 posts or more, then go to their welcoming page and reply.
Once people start posting regularly, I personally forget whether they even had an intro or not, so I don't think its harmful if you never did one. A know a few others mentioned that they didn't. (Cait, maybe??)