Boston received a grade of D-minus in a new report that examined the transparency of spending by the nation’s 30 most-populous cities. The report released Wednesday by Massachusetts Public Interest Research Group gave Boston the unsatisfactory grade largely because it does not publish checkbook-level spending on its website. The most transparent cities were Chicago, New York, and San Francisco, according to the report. Boston ranked sixth from the bottom, a position held by Cleveland. After the release of the report, Mayor Thomas M. Menino’s office released a statement saying the city planned to put its checkbook online in coming months.