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 $64.71   6.719%   $69.0579   $69.06   $69.05   $69.10  
  Purchase #2 $123.38   12.239%   $138.4805   $138.48   $138.50   $138.50  
  Purchase #3 $215.48   13.397%   $244.3479   $244.35   $244.35   $244.30  
  Purchase #4 $156.91   10.856%   $173.9441   $173.94   $173.95   $173.90  
  Purchase #5 $59.81   4.124%   $62.2766   $62.28   $62.30   $62.30  
  Purchase #6 $229.79   8.472%   $249.2578   $249.26   $249.25   $249.30  
  Purchase #7 $81.49   7.103%   $87.2782   $87.28   $87.30   $87.30  
  7 purchase totals = $931.57       $1,024.6430   $1,024.65   $1,024.70   $1,024.70