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 $139.97   6.750%   $149.418   $149.42   $149.40   $149.40  
  Purchase #2 $34.82   6.423%   $37.0565   $37.06   $37.05   $37.10  
  Purchase #3 $158.64   12.654%   $178.7143   $178.71   $178.70   $178.70  
  Purchase #4 $196.38   13.642%   $223.1702   $223.17   $223.15   $223.20  
  Purchase #5 $99.82   8.616%   $108.4205   $108.42   $108.40   $108.40  
  Purchase #6 $142.63   13.286%   $161.5798   $161.58   $161.60   $161.60  
  Purchase #7 $94.40   5.354%   $99.4542   $99.45   $99.45   $99.50  
  7 purchase totals = $866.66       $957.8135   $957.81   $957.75   $957.90