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 $11.57   7.836%   $12.4766   $12.48   $12.50   $12.50  
  Purchase #2 $225.72   10.698%   $249.8675   $249.87   $249.85   $249.90  
  Purchase #3 $148.58   7.686%   $159.9999   $160.00   $160.00   $160.00  
  Purchase #4 $5.34   11.844%   $5.9725   $5.97   $5.95   $6.00  
  Purchase #5 $70.31   8.206%   $76.0796   $76.08   $76.10   $76.10  
  Purchase #6 $25.67   10.190%   $28.2858   $28.29   $28.30   $28.30  
  Purchase #7 $97.61   11.557%   $108.8908   $108.89   $108.90   $108.90  
  7 purchase totals = $584.80       $641.5727   $641.58   $641.60   $641.70