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 $10.42   13.624%   $11.8396   $11.84   $11.85   $11.80  
  Purchase #2 $224.37   11.581%   $250.3543   $250.35   $250.35   $250.40  
  Purchase #3 $220.91   11.681%   $246.7145   $246.71   $246.70   $246.70  
  Purchase #4 $52.85   13.766%   $60.1253   $60.13   $60.15   $60.10  
  Purchase #5 $76.98   13.645%   $87.4839   $87.48   $87.50   $87.50  
  Purchase #6 $209.77   12.621%   $236.2451   $236.25   $236.25   $236.20  
  Purchase #7 $117.84   8.148%   $127.4416   $127.44   $127.45   $127.40  
  7 purchase totals = $913.14       $1,020.2043   $1,020.20   $1,020.25   $1,020.10