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 $28.42   9.680%   $31.1711   $31.17   $31.15   $31.20  
  Purchase #2 $180.90   10.788%   $200.4155   $200.42   $200.40   $200.40  
  Purchase #3 $5.58   5.506%   $5.8872   $5.89   $5.90   $5.90  
  Purchase #4 $185.66   10.720%   $205.5628   $205.56   $205.55   $205.60  
  Purchase #5 $159.98   5.178%   $168.2638   $168.26   $168.25   $168.30  
  Purchase #6 $128.34   8.691%   $139.494   $139.49   $139.50   $139.50  
  Purchase #7 $23.67   13.729%   $26.9197   $26.92   $26.90   $26.90  
  7 purchase totals = $712.55       $777.7141   $777.71   $777.65   $777.80