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 $92.92   6.143%   $98.6281   $98.63   $98.65   $98.60  
  Purchase #2 $236.69   5.851%   $250.5387   $250.54   $250.55   $250.50  
  Purchase #3 $117.73   8.566%   $127.8148   $127.81   $127.80   $127.80  
  Purchase #4 $83.46   9.140%   $91.0882   $91.09   $91.10   $91.10  
  Purchase #5 $22.91   4.186%   $23.869   $23.87   $23.85   $23.90  
  Purchase #6 $178.35   6.416%   $189.7929   $189.79   $189.80   $189.80  
  Purchase #7 $70.52   4.243%   $73.5122   $73.51   $73.50   $73.50  
  7 purchase totals = $802.58       $855.2439   $855.24   $855.25   $855.20