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 $16.60   9.438%   $18.1667   $18.17   $18.15   $18.20  
  Purchase #2 $22.37   5.419%   $23.5822   $23.58   $23.60   $23.60  
  Purchase #3 $95.49   5.635%   $100.8709   $100.87   $100.85   $100.90  
  Purchase #4 $166.58   8.433%   $180.6277   $180.63   $180.65   $180.60  
  Purchase #5 $46.45   11.354%   $51.7239   $51.72   $51.70   $51.70  
  Purchase #6 $83.44   5.219%   $87.7947   $87.79   $87.80   $87.80  
  Purchase #7 $95.67   12.549%   $107.6756   $107.68   $107.70   $107.70  
  7 purchase totals = $526.60       $570.4417   $570.44   $570.45   $570.50