Are you asking about electric or acoustic? It would be helpful if you can name the song.
For example, the verse rhythm section of Metallica's 'Master of Puppets' is supposed to be played with all down strokes. It is a lot easier to play it by alternating up and down strokes but doing so takes off a bit of the aggressive tone of the palm muted down strokes. Another example is the guitarist of the band The Offspring. That dude pretty much strums with down strokes only.
For full chord strumming, try to keep the whole motion smooth and rhythmic, letting your hand swing through moderately loose. Here the difference is a little more subtle between up and down strokes. Still for me, upstrokes tend to be more treble heavy and down strokes tend to be more bass heavy due to my tendency to pull my hand away from the strings as I complete the stroke. Ultimately, the key is to choose a motion that is repeatable and smooth so as to keep the rhythm.
-- My current rig
Guitar: '98 MIJ Ibanez RG520 QS w EMG 81/60
Amp: Laney LC15R Tube, Fernandes 20W SS
Pedals: MI Audio Tube Zone Overdrive, Boss OD3, CS3, MT2, HM2, DS2, CE5, DD7, TU3, Beta Aivin DD2 Delay, Dunlop Crybaby 95Q, Stranger Vol Pedal, BBE SupaCharger
Others: Yamaha RBX 374 Bass, Yamaha MM-6 Synth, M-Audio Fast Track, Shure SM-57, Heet Sound E-Bow Plus
Wanted(pm if selling): EB VPJr/Boss FV500L,Ibanez w Original/Lo-Pro Edge