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 $110.55   5.729%   $116.8834   $116.88   $116.90   $116.90  
  Purchase #2 $16.72   8.541%   $18.1481   $18.15   $18.15   $18.10  
  Purchase #3 $211.94   4.326%   $221.1085   $221.11   $221.10   $221.10  
  Purchase #4 $136.55   6.654%   $145.636   $145.64   $145.65   $145.60  
  Purchase #5 $16.98   12.557%   $19.1122   $19.11   $19.10   $19.10  
  Purchase #6 $40.62   13.460%   $46.0875   $46.09   $46.10   $46.10  
  Purchase #7 $12.86   7.830%   $13.8669   $13.87   $13.85   $13.90  
  7 purchase totals = $546.22       $580.8426   $580.85   $580.85   $580.80