I've always doubled them, but the latest synth set that I've started using has caused me to reconsider. Is it really a common practice in band arranging?
It's certainly common in band music for students and beginners for the euphonium/baritone and trombone parts to be the same; often literally on the same piece of paper with Trombone/Euphonium written in the upper left corner. In more advanced symphonic and brass band music, the instruments are treated independently.
This brass band resource says about the euphonium: "It is equally at home doubling the solo cornet line an octave down, as a melodic or counter-melodic lead, doubling the Bb bass an octave up or occasionally with the 1st trombone." https://www.bandsman.co.uk/writing.htm
So it seems that euphonium/trombone doubling is considered to be an acceptable technique.
It's certainly an acceptable sound. Do it as a matter of course when you're scoring 4-part hymn tunes for an incomplete ensemble. Don't do it when writing more interesting arrangements for a band where you can be confident all chairs will be filled unless you WANT that timbre for a few notes or a few bars.