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 $216.53   12.290%   $243.1415   $243.14   $243.15   $243.10  
  Purchase #2 $91.83   11.660%   $102.5374   $102.54   $102.55   $102.50  
  Purchase #3 $34.75   8.770%   $37.7976   $37.80   $37.80   $37.80  
  Purchase #4 $249.32   9.587%   $273.2223   $273.22   $273.20   $273.20  
  Purchase #5 $165.85   5.761%   $175.4046   $175.40   $175.40   $175.40  
  Purchase #6 $183.55   7.820%   $197.9036   $197.90   $197.90   $197.90  
  Purchase #7 $12.63   6.624%   $13.4666   $13.47   $13.45   $13.50  
  7 purchase totals = $954.46       $1,043.4736   $1,043.47   $1,043.45   $1,043.40