Good question, Keijo. I'll answer it from a Canadian perspective; as I use souvenir sheets on oversized packages to the United States.
The first issue is that the souvenir sheets technically (for the most part) do not pay a given rate. For example,
Canada Post issued a set of flower stamps (Rhododendrons) earlier this year. Two designs each denominated 54¢.
Fifty-four cents is the domestic letter rate. The face value of the souvenir sheet is 2 x 54¢ = $1.08. This does not pay any rate. The closest you could get is by adding a 10¢ stamp to pay the $1.18 domestic oversized rate.
The second issueis the Chinese New Year stamps for 2009. A souvenir sheet was issued with a single $1.65 stamp. The souvenir sheet is small enough to nicely fit on an envelope, and the $1.65 will pay the International letter rate.
So, from a broad perspective, although the $1.08 souvenir sheet does not pay a specific rate, if I use it with other stamps to meet the 100g to 200g U.S. oversized rate of $3.20 on a packet to an American friend technically it is properly used, in-period.
For the technically minded philatelist, the $1.65 souvenir sheet on a letter to any country in the world (other than the United States) would be properly used, paying a prescribed rate that is met by the single usage of the souvenir sheet.

Have I confused you?

David