I worried about this as had lots of stuff I didn't want to wash as it might've been too small & had to be returned!
In the end I took one pack each of 'newborn' & '0-3 months' babygros in - as it turned out we needed the 0-3 size for monster size ds. & using them straight from the (sealed, mothercare) pack seemed to be OK.
I always wash second hand stuff tho' (even tho' most of it comes from a friend who's considerably more fastidious than I am!)