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 $18.96   9.309%   $20.725   $20.73   $20.75   $20.70  
  Purchase #2 $185.64   3.159%   $191.5044   $191.50   $191.50   $191.50  
  Purchase #3 $93.36   12.159%   $104.7116   $104.71   $104.70   $104.70  
  Purchase #4 $69.73   10.663%   $77.1653   $77.17   $77.15   $77.20  
  Purchase #5 $114.39   12.765%   $128.9919   $128.99   $129.00   $129.00  
  Purchase #6 $45.70   12.806%   $51.5523   $51.55   $51.55   $51.60  
  Purchase #7 $98.52   7.391%   $105.8016   $105.80   $105.80   $105.80  
  7 purchase totals = $626.30       $680.4521   $680.45   $680.45   $680.50