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 $101.58   6.512%   $108.1949   $108.19   $108.20   $108.20  
  Purchase #2 $39.39   13.292%   $44.6257   $44.63   $44.65   $44.60  
  Purchase #3 $169.38   13.203%   $191.7432   $191.74   $191.75   $191.70  
  Purchase #4 $94.65   4.206%   $98.631   $98.63   $98.65   $98.60  
  Purchase #5 $57.70   11.193%   $64.1584   $64.16   $64.15   $64.20  
  Purchase #6 $16.69   5.274%   $17.5702   $17.57   $17.55   $17.60  
  Purchase #7 $104.31   11.424%   $116.2264   $116.23   $116.25   $116.20  
  7 purchase totals = $583.70       $641.1498   $641.15   $641.20   $641.10