According to Apple, you are definitely supposed to buy the more expensive full SL installer.
However, it is possible to install SL over Tiger if you buy the $29 update disk. IIRC the only Tiger people that rant into trouble were those who tried to update to SL with the drop-in DVD or the $10 update disk. However, we had several reports of people being able to update Tiger to SL with the $29 disk.