Silicon-based tandem cells are promising to surpass the 29.4% efficiency limit of single-junction silicon solar cells. Silicon heterojunction (SHJ) solar cells are an excellent choice for the bottom cell in tandems because of their record-high open-circuit voltage and excellent spectral efficiency at near-infrared wavelengths. However, the hydrogenated amorphous silicon (aSi:H) properties of SHJ cells have not been optimized for tandem applications. In this work, we vary the front intrinsic and doped a-Si:H layer thicknesses in SHJ cells to study device performance under full-spectrum (300-1200 nm) and infrared-spectrum (700-1200 nm) illumination. We find that the design rules for the front a-Si:H layer stack are surprisingly similar in both cases, but the SHJ cell efficiency variance greatly decreases - by a factor of 3 to 4 - when changing from full-spectrum to infrared-spectrum illumination. That is, cells with substantial a-Si:H thickness variation have narrow efficiency and current distributions when used as bottom cells, which is desirable for subsequent tandem solar module integration. From a manufacturing perspective, there is no need to make additional a-Si:H adjustments specifically for bottom cells, and much higher a-Si:H thickness non-uniformity can be tolerated, which may allow for cheaper deposition tools.