Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $242.95   12.696%   $273.7949   $273.79   $273.80   $273.80  
  Purchase #2 $153.49   10.717%   $169.9395   $169.94   $169.95   $169.90  
  Purchase #3 $127.96   9.534%   $140.1597   $140.16   $140.15   $140.20  
  Purchase #4 $54.57   11.828%   $61.0245   $61.02   $61.00   $61.00  
  Purchase #5 $148.40   10.359%   $163.7728   $163.77   $163.75   $163.80  
  Purchase #6 $98.40   10.733%   $108.9613   $108.96   $108.95   $109.00  
  Purchase #7 $61.79   4.514%   $64.5792   $64.58   $64.60   $64.60  
  7 purchase totals = $887.56       $982.2319   $982.22   $982.20   $982.30