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 $99.88   8.571%   $108.4407   $108.44   $108.45   $108.40  
  Purchase #2 $39.74   6.449%   $42.3028   $42.30   $42.30   $42.30  
  Purchase #3 $146.87   13.829%   $167.1807   $167.18   $167.20   $167.20  
  Purchase #4 $212.62   12.289%   $238.7489   $238.75   $238.75   $238.70  
  Purchase #5 $129.71   11.853%   $145.0845   $145.08   $145.10   $145.10  
  Purchase #6 $20.78   5.597%   $21.9431   $21.94   $21.95   $21.90  
  Purchase #7 $77.93   9.558%   $85.3785   $85.38   $85.40   $85.40  
  7 purchase totals = $727.53       $809.0792   $809.07   $809.15   $809.00