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 $102.42   13.418%   $116.1627   $116.16   $116.15   $116.20  
  Purchase #2 $74.33   8.692%   $80.7908   $80.79   $80.80   $80.80  
  Purchase #3 $173.32   3.873%   $180.0327   $180.03   $180.05   $180.00  
  Purchase #4 $16.74   10.689%   $18.5293   $18.53   $18.55   $18.50  
  Purchase #5 $19.64   5.332%   $20.6872   $20.69   $20.70   $20.70  
  Purchase #6 $60.91   4.128%   $63.4244   $63.42   $63.40   $63.40  
  Purchase #7 $92.45   6.806%   $98.7421   $98.74   $98.75   $98.70  
  7 purchase totals = $539.81       $578.3692   $578.36   $578.40   $578.30