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 $25.45   13.629%   $28.9186   $28.92   $28.90   $28.90  
  Purchase #2 $37.44   7.696%   $40.3214   $40.32   $40.30   $40.30  
  Purchase #3 $61.48   7.204%   $65.909   $65.91   $65.90   $65.90  
  Purchase #4 $110.93   13.668%   $126.0919   $126.09   $126.10   $126.10  
  Purchase #5 $104.84   10.853%   $116.2183   $116.22   $116.20   $116.20  
  Purchase #6 $185.25   8.641%   $201.2575   $201.26   $201.25   $201.30  
  Purchase #7 $12.66   5.706%   $13.3824   $13.38   $13.40   $13.40  
  7 purchase totals = $538.05       $592.0991   $592.10   $592.05   $592.10