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 $140.45   12.244%   $157.6467   $157.65   $157.65   $157.60  
  Purchase #2 $183.47   3.104%   $189.1649   $189.16   $189.15   $189.20  
  Purchase #3 $186.90   5.384%   $196.9627   $196.96   $196.95   $197.00  
  Purchase #4 $212.71   12.811%   $239.9603   $239.96   $239.95   $240.00  
  Purchase #5 $151.84   9.471%   $166.2208   $166.22   $166.20   $166.20  
  Purchase #6 $24.86   9.383%   $27.1926   $27.19   $27.20   $27.20  
  Purchase #7 $34.64   4.383%   $36.1583   $36.16   $36.15   $36.20  
  7 purchase totals = $934.87       $1,013.3063   $1,013.30   $1,013.25   $1,013.40