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 $26.59   6.734%   $28.3806   $28.38   $28.40   $28.40  
  Purchase #2 $14.90   6.148%   $15.8161   $15.82   $15.80   $15.80  
  Purchase #3 $148.57   13.365%   $168.4264   $168.43   $168.45   $168.40  
  Purchase #4 $105.65   3.792%   $109.6562   $109.66   $109.65   $109.70  
  Purchase #5 $43.54   7.254%   $46.6984   $46.70   $46.70   $46.70  
  Purchase #6 $71.64   6.398%   $76.2235   $76.22   $76.20   $76.20  
  Purchase #7 $177.61   12.187%   $199.2553   $199.26   $199.25   $199.30  
  7 purchase totals = $588.50       $644.4565   $644.47   $644.45   $644.50