In general I think we should reserve the tag accountants for CPAs/Chartered/Certified Accountants who offer a broad range of accounting services.

An alternative might be bookkeeper, but I would go with the more general office=financial_services, which potentially could cover the two former options as well, but is best reserved for things which are either rare or (currently) awkward to categorise. This can be suitably subtagged with a straight financial_services=tax_preparation or financial_services:type=tax_preparation.

Given the sheer number of H&R Block offices a straight tag office=tax_preparation might be justified.