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 $30.38   6.516%   $32.3596   $32.36   $32.35   $32.40  
  Purchase #2 $58.76   4.870%   $61.6216   $61.62   $61.60   $61.60  
  Purchase #3 $38.25   13.729%   $43.5013   $43.50   $43.50   $43.50  
  Purchase #4 $249.67   12.442%   $280.7339   $280.73   $280.75   $280.70  
  Purchase #5 $61.86   6.644%   $65.97   $65.97   $65.95   $66.00  
  Purchase #6 $113.47   9.485%   $124.2326   $124.23   $124.25   $124.20  
  Purchase #7 $177.44   4.869%   $186.0796   $186.08   $186.10   $186.10  
  7 purchase totals = $729.83       $794.4986   $794.49   $794.50   $794.50