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 $190.54   12.572%   $214.4947   $214.49   $214.50   $214.50  
  Purchase #2 $163.68   10.389%   $180.6847   $180.68   $180.70   $180.70  
  Purchase #3 $36.70   11.594%   $40.955   $40.96   $40.95   $41.00  
  Purchase #4 $224.60   13.645%   $255.2467   $255.25   $255.25   $255.20  
  Purchase #5 $13.73   12.116%   $15.3935   $15.39   $15.40   $15.40  
  Purchase #6 $121.97   13.477%   $138.4079   $138.41   $138.40   $138.40  
  Purchase #7 $154.94   9.785%   $170.1009   $170.10   $170.10   $170.10  
  7 purchase totals = $906.16       $1,015.2834   $1,015.28   $1,015.30   $1,015.30