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 $246.58   4.612%   $257.9523   $257.95   $257.95   $258.00  
  Purchase #2 $119.96   8.369%   $129.9995   $130.00   $130.00   $130.00  
  Purchase #3 $250.65   6.190%   $266.1652   $266.17   $266.15   $266.20  
  Purchase #4 $249.73   10.818%   $276.7458   $276.75   $276.75   $276.70  
  Purchase #5 $126.55   9.659%   $138.7735   $138.77   $138.75   $138.80  
  Purchase #6 $209.82   12.281%   $235.588   $235.59   $235.60   $235.60  
  Purchase #7 $185.71   9.597%   $203.5326   $203.53   $203.55   $203.50  
  7 purchase totals = $1,389.00       $1,508.7569   $1,508.76   $1,508.75   $1,508.80