I almost always mark the mailed immediately after I print the wrapper, and I use PBS postage and DC. I tend to normally wrap books that need to go out at night when I get home from work. I then put them in a bag at the door, and I take the bag with me when I leave for work in the morning. I have a blue box out front of the local post office that I put them in on my way. I personally see no reason to drive to the post office after 5 when I know the pick up has already been done for the day, and they'll just sit there until the next day anyway. My books will leave at the exact same time as the member who put theirs in there last night then properly marked the mailed.
I realize something could happen between that night and that morning in regards to me mailing them out, but it's easier for me to do it all at once. Plus, I don't want to run the risk of it timing out. And since it would take some sort of catastrophic event to prevent me (or DH) from making it to the post office (sometime before 5 that day or even the next day), then I'm sure whatever is going on in my life is going to be more important to me at that time than the book(s) by my door. It would never be my intention, but I would take a hit on my account for mailing late if I had something major going on.