I think using a pawn as a value of 1 always makes sense, but to be more specific (same as normal chess) it should be assumed that it is a normal pawn in the starting position, with other pawns still next to it.
Pawns can change value in a game. A pawn on the 7th rank is often valued about the same as a rook. Isolated pawns lose value. And in infinite chess I think the hawk's pawns are worth much less at the start. But for general discussion, I think it is almost always assumed that a normal pawn has a value of "one".
(fyi: some programmers use centipawns, i.e. 1 pawn = 100 centipawns).
@captaintugwash
Sorry, that was a typo. I meant to say that pawns should be the base value you judge by and should be assigned the value of 1 (for humans.)
As you point out, values change on board position and games state, requiring reevaluation. A computer is likely to want to assign a value per piece and temporally (for example to assign a value to a pawn based on its potential future value because it's a passed pawn nearing promotion.)
I typed hastily, sorry for that.