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 $11.99   3.349%   $12.3915   $12.39   $12.40   $12.40  
  Purchase #2 $177.65   5.555%   $187.5185   $187.52   $187.50   $187.50  
  Purchase #3 $131.62   10.496%   $145.4348   $145.43   $145.45   $145.40  
  Purchase #4 $8.32   9.372%   $9.0998   $9.10   $9.10   $9.10  
  Purchase #5 $173.49   9.540%   $190.0409   $190.04   $190.05   $190.00  
  Purchase #6 $24.29   7.813%   $26.1878   $26.19   $26.20   $26.20  
  Purchase #7 $15.99   12.400%   $17.9728   $17.97   $17.95   $18.00  
  7 purchase totals = $543.35       $588.6461   $588.64   $588.65   $588.60