The imbalance and excess loss in multimode interference couplers with fabrication errors are examined. Remarkably, there exists a number of optimum access waveguide widths which give a minimum imbalance. Furthermore, quite low excess loss can be simultaneously achieved by choosing one particular optimum width. It is also shown that paired interference couplers have better imbalance performance than general interference couplers, given the same fabrication tolerances and length constraint for both coupler types. In particular, a paired interference coupler with an access waveguide width approximately 0.3 times the coupler width has the best imbalance performance. These results are obtained using a simple model and are supported by more sophisticated numerical models and experimental data from couplers fabricated in InP/InGaAsP.