I seal all mine. I just take a tape gun with me to seal them back up if they open them. Customs will seal them back up for you, but they use the "opened by customs" tape, so I prefer to do it myself.
Just make sure you have invoices with the contents of each package, otherwise they can make you open every package and identify the contents.
The more you go through, the more they will get to know you and you may find they use a little less vigilance after a while.
Rob