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 $9.75   6.780%   $10.4111   $10.41   $10.40   $10.40  
  Purchase #2 $9.99   13.237%   $11.3124   $11.31   $11.30   $11.30  
  Purchase #3 $107.90   4.804%   $113.0835   $113.08   $113.10   $113.10  
  Purchase #4 $31.44   8.661%   $34.163   $34.16   $34.15   $34.20  
  Purchase #5 $72.50   11.440%   $80.794   $80.79   $80.80   $80.80  
  Purchase #6 $183.77   3.130%   $189.522   $189.52   $189.50   $189.50  
  Purchase #7 $127.61   10.652%   $141.203   $141.20   $141.20   $141.20  
  7 purchase totals = $542.96       $580.4890   $580.47   $580.45   $580.50