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 $18.38   13.772%   $20.9113   $20.91   $20.90   $20.90  
  Purchase #2 $121.99   12.734%   $137.5242   $137.52   $137.50   $137.50  
  Purchase #3 $12.83   13.303%   $14.5368   $14.54   $14.55   $14.50  
  Purchase #4 $21.45   7.602%   $23.0806   $23.08   $23.10   $23.10  
  Purchase #5 $42.43   7.641%   $45.6721   $45.67   $45.65   $45.70  
  Purchase #6 $182.56   8.654%   $198.3587   $198.36   $198.35   $198.40  
  Purchase #7 $128.89   6.444%   $137.1957   $137.20   $137.20   $137.20  
  7 purchase totals = $528.53       $577.2794   $577.28   $577.25   $577.30