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 $236.72   8.461%   $256.7489   $256.75   $256.75   $256.70  
  Purchase #2 $68.61   7.220%   $73.5636   $73.56   $73.55   $73.60  
  Purchase #3 $11.39   10.643%   $12.6022   $12.60   $12.60   $12.60  
  Purchase #4 $63.42   10.831%   $70.289   $70.29   $70.30   $70.30  
  Purchase #5 $130.82   5.706%   $138.2846   $138.28   $138.30   $138.30  
  Purchase #6 $43.62   11.491%   $48.6324   $48.63   $48.65   $48.60  
  Purchase #7 $15.97   11.200%   $17.7586   $17.76   $17.75   $17.80  
  7 purchase totals = $570.55       $617.8793   $617.87   $617.90   $617.90