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 $217.87   13.367%   $246.9927   $246.99   $247.00   $247.00  
  Purchase #2 $153.52   12.814%   $173.1921   $173.19   $173.20   $173.20  
  Purchase #3 $134.80   3.835%   $139.9696   $139.97   $139.95   $140.00  
  Purchase #4 $193.91   11.800%   $216.7914   $216.79   $216.80   $216.80  
  Purchase #5 $238.78   4.690%   $249.9788   $249.98   $250.00   $250.00  
  Purchase #6 $148.91   8.801%   $162.0156   $162.02   $162.00   $162.00  
  Purchase #7 $159.92   6.791%   $170.7802   $170.78   $170.80   $170.80  
  7 purchase totals = $1,247.71       $1,359.7204   $1,359.72   $1,359.75   $1,359.80