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 $11.67   6.753%   $12.4581   $12.46   $12.45   $12.50  
  Purchase #2 $236.50   5.147%   $248.6727   $248.67   $248.65   $248.70  
  Purchase #3 $132.71   9.146%   $144.8477   $144.85   $144.85   $144.80  
  Purchase #4 $240.34   4.357%   $250.8116   $250.81   $250.80   $250.80  
  Purchase #5 $94.93   13.731%   $107.9648   $107.96   $107.95   $108.00  
  Purchase #6 $189.91   12.228%   $213.1322   $213.13   $213.15   $213.10  
  Purchase #7 $89.92   12.800%   $101.4298   $101.43   $101.45   $101.40  
  7 purchase totals = $995.98       $1,079.3169   $1,079.31   $1,079.30   $1,079.30