I use Lucks and think they're a good compromise between Kopykakes and Icing Images Premium. I had major problems getting Icing Images sheets to meld with the buttercream. I don't have that problem with Lucks. Lucks are heavier and more pliable than KK. So, for me at least, they're a good middle of the road choice.
But as to the original question...I've printed them weeks ahead of time. Back when I had a balky printer, I felt I had to do that just in case I had to turn to someone else to do the printing. Didn't want to be searching for help at the last minute. Kept them in a large ziploc, OUT OF THE LIGHT, and they were fine.
The light issue is a biggie! A year or so ago, I had a rather large cake with a collage of photos. I finished it and put it on the dining room table while I did other things. I was horrified to find that a tiny sliver of sunlight from a decorative window had danced across the top of the cake during the afternoon. Everywhere it touched the cake, the color was drastically changed. I had to scrape everything off, reprint (thank goodness that was a day my printer decided to WORK) and redecorate the whole thing. Big lesson learned.
So, yeah..print 'em early. Keep them sealed in bags. Store in the dark!