I dps when I feel comfortable doing so.
However, as others have explained, I came from two MMO's in particular (WoW and EQ) where if you were DPSing as a healer you were told to GTFO.
On a side note, in those games if a healer needed to DPS, then it meant your DPS classes were rather lackluster. I personally take that aspect into FFXIV. The DPS classes should be able to kill mobs fast enough to where a healer should never need to DPS.
Just saw a post above here, I am also on console/controller user. It is not overly difficult, but definitely more difficult than a mouse/ keyboard user to switch CS on and off.