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 $28.79   9.685%   $31.5783   $31.58   $31.60   $31.60  
  Purchase #2 $132.34   12.142%   $148.4087   $148.41   $148.40   $148.40  
  Purchase #3 $198.67   5.332%   $209.2631   $209.26   $209.25   $209.30  
  Purchase #4 $16.88   7.131%   $18.0837   $18.08   $18.10   $18.10  
  Purchase #5 $28.88   6.338%   $30.7104   $30.71   $30.70   $30.70  
  Purchase #6 $151.83   11.749%   $169.6685   $169.67   $169.65   $169.70  
  Purchase #7 $133.41   8.396%   $144.6111   $144.61   $144.60   $144.60  
  7 purchase totals = $690.80       $752.3238   $752.32   $752.30   $752.40