I have seen it work reliably without, but you are correct it may be needed.
Testing at my house here in the country, 2200sq ft, 2-phase panel, about 160ft to transformer on the pole, no need for phase coupler. In the city with typical street grids of houses, no need. But I just finished helping with a large country home that had a main panel and 3 sub-panels. Needless to say phase couplers were definitely required.
Anyone considering making SAI a regular part of your work, PM me and we can cover specifics.