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 $18.60   7.403%   $19.977   $19.98   $20.00   $20.00  
  Purchase #2 $10.25   4.470%   $10.7082   $10.71   $10.70   $10.70  
  Purchase #3 $171.33   6.525%   $182.5093   $182.51   $182.50   $182.50  
  Purchase #4 $200.67   6.452%   $213.6172   $213.62   $213.60   $213.60  
  Purchase #5 $80.25   3.374%   $82.9576   $82.96   $82.95   $83.00  
  Purchase #6 $99.44   6.330%   $105.7346   $105.73   $105.75   $105.70  
  Purchase #7 $152.41   7.243%   $163.4491   $163.45   $163.45   $163.40  
  7 purchase totals = $732.95       $778.9530   $778.96   $778.95   $778.90