The goal of this study is to investigate the tradeoff in the design of anti-scatter grid for a photon-counting detector. The anti-scatter grid can block the primary beam at the boundary of each detector pixel, which reduces the charge sharing. On the other hand, thick anti-scatter grids also reduce the detector fill factor and thus reduce dose efficiency. To quantitatively evaluate the performance of spectral imaging, we use both CRLB-based metrics from analytic calculation and CNR-based metrics from projection-based simulation. The results indicate that the use of anti-scatter grid can reduce the charge sharing, but its dose penalty in geometric efficiency outweighs the dose benefit in spectral separation. Therefore, thinner anti-scatter grid plates are preferred for photon-counting detectors in both single-energy and spectral imaging.