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 $163.99   4.147%   $170.7907   $170.79   $170.80   $170.80  
  Purchase #2 $55.27   8.697%   $60.0768   $60.08   $60.10   $60.10  
  Purchase #3 $60.84   9.311%   $66.5048   $66.50   $66.50   $66.50  
  Purchase #4 $127.87   9.778%   $140.3731   $140.37   $140.35   $140.40  
  Purchase #5 $108.33   6.839%   $115.7387   $115.74   $115.75   $115.70  
  Purchase #6 $72.91   6.228%   $77.4508   $77.45   $77.45   $77.50  
  Purchase #7 $10.89   9.463%   $11.9205   $11.92   $11.90   $11.90  
  7 purchase totals = $600.10       $642.8554   $642.85   $642.85   $642.90