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 $134.92   6.646%   $143.8868   $143.89   $143.90   $143.90  
  Purchase #2 $95.50   11.279%   $106.2714   $106.27   $106.25   $106.30  
  Purchase #3 $23.68   11.400%   $26.3795   $26.38   $26.40   $26.40  
  Purchase #4 $159.59   10.781%   $176.7954   $176.80   $176.80   $176.80  
  Purchase #5 $100.41   8.266%   $108.7099   $108.71   $108.70   $108.70  
  Purchase #6 $46.39   12.333%   $52.1113   $52.11   $52.10   $52.10  
  Purchase #7 $27.91   7.689%   $30.056   $30.06   $30.05   $30.10  
  7 purchase totals = $588.40       $644.2103   $644.22   $644.20   $644.30