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 $249.49   8.472%   $270.6268   $270.63   $270.65   $270.60  
  Purchase #2 $152.42   13.864%   $173.5515   $173.55   $173.55   $173.60  
  Purchase #3 $76.86   11.568%   $85.7512   $85.75   $85.75   $85.80  
  Purchase #4 $127.32   11.258%   $141.6537   $141.65   $141.65   $141.70  
  Purchase #5 $184.66   8.573%   $200.4909   $200.49   $200.50   $200.50  
  Purchase #6 $30.78   7.680%   $33.1439   $33.14   $33.15   $33.10  
  Purchase #7 $60.30   4.467%   $62.9936   $62.99   $63.00   $63.00  
  7 purchase totals = $881.83       $968.2116   $968.20   $968.25   $968.30