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 $241.73   13.104%   $273.4063   $273.41   $273.40   $273.40  
  Purchase #2 $117.50   3.523%   $121.6395   $121.64   $121.65   $121.60  
  Purchase #3 $38.57   8.205%   $41.7347   $41.73   $41.75   $41.70  
  Purchase #4 $50.44   13.312%   $57.1546   $57.15   $57.15   $57.20  
  Purchase #5 $200.82   9.360%   $219.6168   $219.62   $219.60   $219.60  
  Purchase #6 $112.43   10.847%   $124.6253   $124.63   $124.65   $124.60  
  Purchase #7 $125.73   3.787%   $130.4914   $130.49   $130.50   $130.50  
  7 purchase totals = $887.22       $968.6686   $968.67   $968.70   $968.60