I know that the IVW (Dutch CAA) does not allow fixed de-rates.
Can't speak for other types, but they (Dutch CAA/IVW) do allow it on the 744.
At my company we use fixed derates TO 1 & 2, and derate those using assumed temp method, if possible.
This gives a lot of flexibility on short or contaminated rwy's, and allows a lot of derate on long dry rwy's.
We also use 744's that don't have the fixed derates, and derate from the max power setting. This reduces the maximum deration considerably.