No, they have to be on the front. Believe it or not, back in the early days, USK logos actually USED to be on the back (or on the side for big box PC games) before all these suicide shootings and media drama pointing the finger towards video games. Then they were moved to the front and then in 2009 they made them HUGE until they realized they were butt ugly and changed their design to the mildly less offensive ones we have now.
Just be glad they don't look like this anymore:
*yuck*
I just checked my collection and I have a couple of German editions with those old logos. And quite a lot of other games that have been "affected" by this plague as well.
Baldur's Gate Dark Alliance 2 (PS2), Mega Man: Powered Up (PSP), Ikaruga (NGC) and Soul Reaver 2 have a small USK logo.
My copies of Castlevania (PS2), Castlevania: Curse of Darkness (PS2), Silent Hill 2 (PS2), God Hand (PS2), Metal Gear Ac!d (PSP), Tales of Xillia LE (PS3), Game & Watch Collection (NDS), Mario Kart Wii (Wii), Monster Hunter Tri (Wii), New Super Mario Bros (Wii), Pandora's Tower (Wii), Silent Hill: Shattered Memories (Wii), The Last Story (Wii), The Legend of Zelda: Skyward Sword (Wii), Xenoblade Chronicles (Wii), Torchlight II (PC) and Siren: Blood Curse (PS3) all have the current size one, and some of them the old style one from the Chrono Trigger box. Why...?
Fortunately Devil Summoner: Raidou Kuzunoha vs the Soulless Army (PS2) and Maken Shao - Demon Sword (PS2) only have a USK sticker on them actually.
Good lord, I never realised it was this bad! :-(