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 $16.63   9.289%   $18.1748   $18.17   $18.15   $18.20  
  Purchase #2 $52.36   11.664%   $58.4673   $58.47   $58.45   $58.50  
  Purchase #3 $105.59   12.393%   $118.6758   $118.68   $118.70   $118.70  
  Purchase #4 $170.62   9.262%   $186.4228   $186.42   $186.40   $186.40  
  Purchase #5 $144.44   10.617%   $159.7752   $159.78   $159.80   $159.80  
  Purchase #6 $37.71   5.323%   $39.7173   $39.72   $39.70   $39.70  
  Purchase #7 $59.86   9.178%   $65.354   $65.35   $65.35   $65.40  
  7 purchase totals = $587.21       $646.5872   $646.59   $646.55   $646.70