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 $243.42   5.806%   $257.553   $257.55   $257.55   $257.60  
  Purchase #2 $83.93   11.491%   $93.5744   $93.57   $93.55   $93.60  
  Purchase #3 $95.27   12.428%   $107.1102   $107.11   $107.10   $107.10  
  Purchase #4 $116.99   13.867%   $133.213   $133.21   $133.20   $133.20  
  Purchase #5 $113.57   9.232%   $124.0548   $124.05   $124.05   $124.10  
  Purchase #6 $23.54   13.301%   $26.6711   $26.67   $26.65   $26.70  
  Purchase #7 $91.87   5.823%   $97.2196   $97.22   $97.20   $97.20  
  7 purchase totals = $768.59       $839.3961   $839.38   $839.30   $839.50