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 $19.40   8.617%   $21.0717   $21.07   $21.05   $21.10  
  Purchase #2 $168.40   12.215%   $188.9701   $188.97   $188.95   $189.00  
  Purchase #3 $50.85   5.574%   $53.6844   $53.68   $53.70   $53.70  
  Purchase #4 $47.52   6.292%   $50.51   $50.51   $50.50   $50.50  
  Purchase #5 $175.36   10.530%   $193.8254   $193.83   $193.85   $193.80  
  Purchase #6 $132.88   10.204%   $146.4391   $146.44   $146.45   $146.40  
  Purchase #7 $73.74   7.651%   $79.3818   $79.38   $79.40   $79.40  
  7 purchase totals = $668.15       $733.8825   $733.88   $733.90   $733.90