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 $244.61   13.169%   $276.8227   $276.82   $276.80   $276.80  
  Purchase #2 $95.26   7.422%   $102.3302   $102.33   $102.35   $102.30  
  Purchase #3 $183.70   4.447%   $191.8691   $191.87   $191.85   $191.90  
  Purchase #4 $162.55   9.160%   $177.4396   $177.44   $177.45   $177.40  
  Purchase #5 $59.68   8.450%   $64.723   $64.72   $64.70   $64.70  
  Purchase #6 $245.50   6.658%   $261.8454   $261.85   $261.85   $261.80  
  Purchase #7 $201.95   7.683%   $217.4658   $217.47   $217.45   $217.50  
  7 purchase totals = $1,193.25       $1,292.4958   $1,292.50   $1,292.45   $1,292.40