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 $6.50   5.304%   $6.8448   $6.84   $6.85   $6.80  
  Purchase #2 $103.80   10.848%   $115.0602   $115.06   $115.05   $115.10  
  Purchase #3 $190.81   10.606%   $211.0473   $211.05   $211.05   $211.00  
  Purchase #4 $151.87   4.238%   $158.3063   $158.31   $158.30   $158.30  
  Purchase #5 $109.37   11.273%   $121.6993   $121.70   $121.70   $121.70  
  Purchase #6 $140.55   13.577%   $159.6325   $159.63   $159.65   $159.60  
  Purchase #7 $153.77   4.546%   $160.7604   $160.76   $160.75   $160.80  
  7 purchase totals = $856.67       $933.3508   $933.35   $933.35   $933.30