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 $19.45   4.330%   $20.2922   $20.29   $20.30   $20.30  
  Purchase #2 $14.99   10.826%   $16.6128   $16.61   $16.60   $16.60  
  Purchase #3 $159.80   13.499%   $181.3714   $181.37   $181.35   $181.40  
  Purchase #4 $183.27   12.778%   $206.6882   $206.69   $206.70   $206.70  
  Purchase #5 $62.37   7.639%   $67.1344   $67.13   $67.15   $67.10  
  Purchase #6 $127.43   13.126%   $144.1565   $144.16   $144.15   $144.20  
  Purchase #7 $43.39   4.571%   $45.3734   $45.37   $45.35   $45.40  
  7 purchase totals = $610.70       $681.6289   $681.62   $681.60   $681.70