As mentioned above, if your blade is Edo period or later, (unless it's in Shin-Gunto mounts) the chances it was ever used to kill are quite low. And even if it's Koto, remember that the primary killers on a Japanese battlefield back then would have been Bow and arrows, Yari and Naginata, or towards the later period, Tanegashima (matchlock muskets). Swords were usually secondary weapons.
A blade in Shin-Gunto mounts makes the kill probability higher, as WW2-era Japanese officers were rather keen on "testing" their blades, usually on the necks of helpless prisoners. But if your blade is coming out of Japan, the previous owner may never have been deployed overseas.
This will sound "Hippy-Dippy", but hey, I'm from California... If your wife is that concerned, you can burn some sage or incense next to the blade while playing some Japanese Temple Music or Mantras via YouTube and such. Just a little karmic cleansing to smooth things over if the blade's Kami is a little shook by the move from good ol' Nippon to the Lone Star State. I do it with all my blades.