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 $88.51   13.478%   $100.4394   $100.44   $100.45   $100.40  
  Purchase #2 $63.30   4.714%   $66.284   $66.28   $66.30   $66.30  
  Purchase #3 $103.27   12.585%   $116.2665   $116.27   $116.25   $116.30  
  Purchase #4 $121.58   11.112%   $135.09   $135.09   $135.10   $135.10  
  Purchase #5 $128.40   12.550%   $144.5142   $144.51   $144.50   $144.50  
  Purchase #6 $13.46   12.874%   $15.1928   $15.19   $15.20   $15.20  
  Purchase #7 $58.58   4.448%   $61.1856   $61.19   $61.20   $61.20  
  7 purchase totals = $577.10       $638.9725   $638.97   $639.00   $639.00