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 $39.34   7.150%   $42.1528   $42.15   $42.15   $42.20  
  Purchase #2 $88.36   5.829%   $93.5105   $93.51   $93.50   $93.50  
  Purchase #3 $19.38   6.821%   $20.7019   $20.70   $20.70   $20.70  
  Purchase #4 $189.62   9.610%   $207.8425   $207.84   $207.85   $207.80  
  Purchase #5 $117.62   12.284%   $132.0684   $132.07   $132.05   $132.10  
  Purchase #6 $178.81   5.661%   $188.9324   $188.93   $188.95   $188.90  
  Purchase #7 $150.25   3.623%   $155.6936   $155.69   $155.70   $155.70  
  7 purchase totals = $783.38       $840.9021   $840.89   $840.90   $840.90