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 $204.77   6.599%   $218.2828   $218.28   $218.30   $218.30  
  Purchase #2 $129.78   5.214%   $136.5467   $136.55   $136.55   $136.50  
  Purchase #3 $72.52   4.665%   $75.9031   $75.90   $75.90   $75.90  
  Purchase #4 $213.51   9.360%   $233.4945   $233.49   $233.50   $233.50  
  Purchase #5 $164.35   13.337%   $186.2694   $186.27   $186.25   $186.30  
  Purchase #6 $187.76   10.493%   $207.4617   $207.46   $207.45   $207.50  
  Purchase #7 $103.81   13.496%   $117.8202   $117.82   $117.80   $117.80  
  7 purchase totals = $1,076.50       $1,175.7784   $1,175.77   $1,175.75   $1,175.80