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 $18.99   6.798%   $20.2809   $20.28   $20.30   $20.30  
  Purchase #2 $138.80   8.394%   $150.4509   $150.45   $150.45   $150.50  
  Purchase #3 $149.90   12.394%   $168.4786   $168.48   $168.50   $168.50  
  Purchase #4 $107.67   4.350%   $112.3536   $112.35   $112.35   $112.40  
  Purchase #5 $83.41   6.429%   $88.7724   $88.77   $88.75   $88.80  
  Purchase #6 $10.62   13.825%   $12.0882   $12.09   $12.10   $12.10  
  Purchase #7 $12.31   4.184%   $12.8251   $12.83   $12.85   $12.80  
  7 purchase totals = $521.70       $565.2497   $565.25   $565.30   $565.40