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 $24.61   12.301%   $27.6373   $27.64   $27.65   $27.60  
  Purchase #2 $68.84   11.578%   $76.8103   $76.81   $76.80   $76.80  
  Purchase #3 $74.95   8.389%   $81.2376   $81.24   $81.25   $81.20  
  Purchase #4 $240.44   5.452%   $253.5488   $253.55   $253.55   $253.50  
  Purchase #5 $198.55   9.395%   $217.2038   $217.20   $217.20   $217.20  
  Purchase #6 $146.29   3.863%   $151.9412   $151.94   $151.95   $151.90  
  Purchase #7 $132.78   9.514%   $145.4127   $145.41   $145.40   $145.40  
  7 purchase totals = $886.46       $953.7917   $953.79   $953.80   $953.60