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 $246.43   12.240%   $276.593   $276.59   $276.60   $276.60  
  Purchase #2 $243.72   4.844%   $255.5258   $255.53   $255.55   $255.50  
  Purchase #3 $43.84   6.669%   $46.7637   $46.76   $46.75   $46.80  
  Purchase #4 $130.49   10.821%   $144.6103   $144.61   $144.60   $144.60  
  Purchase #5 $181.32   12.859%   $204.6359   $204.64   $204.65   $204.60  
  Purchase #6 $219.86   9.665%   $241.1095   $241.11   $241.10   $241.10  
  Purchase #7 $115.33   3.792%   $119.7033   $119.70   $119.70   $119.70  
  7 purchase totals = $1,180.99       $1,288.9415   $1,288.94   $1,288.95   $1,288.90