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 $70.99   13.525%   $80.5914   $80.59   $80.60   $80.60  
  Purchase #2 $113.34   10.794%   $125.5739   $125.57   $125.55   $125.60  
  Purchase #3 $250.67   8.808%   $272.749   $272.75   $272.75   $272.70  
  Purchase #4 $158.25   8.168%   $171.1759   $171.18   $171.20   $171.20  
  Purchase #5 $62.82   9.259%   $68.6365   $68.64   $68.65   $68.60  
  Purchase #6 $97.74   9.827%   $107.3449   $107.34   $107.35   $107.30  
  Purchase #7 $127.71   11.127%   $141.9203   $141.92   $141.90   $141.90  
  7 purchase totals = $881.52       $967.9919   $967.99   $968.00   $967.90