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 $242.44   6.207%   $257.4883   $257.49   $257.50   $257.50  
  Purchase #2 $35.78   11.382%   $39.8525   $39.85   $39.85   $39.90  
  Purchase #3 $5.28   6.752%   $5.6365   $5.64   $5.65   $5.60  
  Purchase #4 $111.84   5.414%   $117.895   $117.90   $117.90   $117.90  
  Purchase #5 $73.54   9.631%   $80.6226   $80.62   $80.60   $80.60  
  Purchase #6 $230.97   12.128%   $258.982   $258.98   $259.00   $259.00  
  Purchase #7 $249.91   5.162%   $262.8104   $262.81   $262.80   $262.80  
  7 purchase totals = $949.76       $1,023.2873   $1,023.29   $1,023.30   $1,023.30