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 $59.53   6.655%   $63.4917   $63.49   $63.50   $63.50  
  Purchase #2 $97.30   5.478%   $102.6301   $102.63   $102.65   $102.60  
  Purchase #3 $232.55   10.485%   $256.9329   $256.93   $256.95   $256.90  
  Purchase #4 $110.76   5.671%   $117.0412   $117.04   $117.05   $117.00  
  Purchase #5 $172.79   13.427%   $195.9905   $195.99   $196.00   $196.00  
  Purchase #6 $127.44   13.423%   $144.5463   $144.55   $144.55   $144.50  
  Purchase #7 $108.73   12.764%   $122.6083   $122.61   $122.60   $122.60  
  7 purchase totals = $909.10       $1,003.2410   $1,003.24   $1,003.30   $1,003.10