I like to send my Welcome email 2-3 days before check-in, for the following reasons:
1) When the trip draws nearer, guests tend to get a little anxious and want to make sure they have everything they need to arrive at and check in to my place
2) Some guests like to print out the email and take it with them in case their online connection becomes unavailable. There are enough details to worry about on the day of travel to also have to remember to print it out.
3) I used to send the email the day before check-in, but quite a few guests would contact me 2-3 days before their trip, asking for the address, directions, check-in steps, etc., so now I send it a day or two sooner.
Additionally, the custom code for our guests does not become activated until check-in time, so security isn’t an issue.