楼上说的对,once一般情况下不需要倒装,不管是作为连词、副词还是名词；
但是,在以否定词开头的情况下需要倒装,如
Not once did he talk to me.
他一次也没有和我谈过.
The old couple have been married for 40 years and never once have they quarreled with each other.
这对老夫妻结婚40年间连一次吵架都没有.