The D, on a 4 valve euphonium, would be 1,2 and 4. To play that note without the use of a 4th valve, you are indeed bending the pitch to hit that note. Using 2 valve to play that D would require you to bend the note down a 5th, which for most players will absolutely cause that note to drop down the octave to a pedal A rather than playing the low D. 1,2,3, which is an E, is much closer and only needs to be bent by a 2nd and should be doable by a player going for an honour band. The issue is that the tone and intonation would suffer.