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 $165.58   11.547%   $184.6995   $184.70   $184.70   $184.70  
  Purchase #2 $103.63   12.544%   $116.6293   $116.63   $116.65   $116.60  
  Purchase #3 $118.41   8.868%   $128.9106   $128.91   $128.90   $128.90  
  Purchase #4 $111.75   8.689%   $121.46   $121.46   $121.45   $121.50  
  Purchase #5 $91.42   6.133%   $97.0268   $97.03   $97.05   $97.00  
  Purchase #6 $134.90   7.316%   $144.7693   $144.77   $144.75   $144.80  
  Purchase #7 $199.31   3.482%   $206.25   $206.25   $206.25   $206.30  
  7 purchase totals = $925.00       $999.7455   $999.75   $999.75   $999.80