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 $140.76   12.528%   $158.3944   $158.39   $158.40   $158.40  
  Purchase #2 $28.87   4.560%   $30.1865   $30.19   $30.20   $30.20  
  Purchase #3 $220.74   3.492%   $228.4482   $228.45   $228.45   $228.40  
  Purchase #4 $111.50   13.827%   $126.9171   $126.92   $126.90   $126.90  
  Purchase #5 $213.85   7.414%   $229.7048   $229.70   $229.70   $229.70  
  Purchase #6 $44.93   12.695%   $50.6339   $50.63   $50.65   $50.60  
  Purchase #7 $79.44   7.337%   $85.2685   $85.27   $85.25   $85.30  
  7 purchase totals = $840.09       $909.5534   $909.55   $909.55   $909.50