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 $12.32   9.451%   $13.4844   $13.48   $13.50   $13.50  
  Purchase #2 $5.89   13.174%   $6.6659   $6.67   $6.65   $6.70  
  Purchase #3 $8.74   10.397%   $9.6487   $9.65   $9.65   $9.60  
  Purchase #4 $103.70   10.801%   $114.9006   $114.90   $114.90   $114.90  
  Purchase #5 $118.71   4.751%   $124.3499   $124.35   $124.35   $124.30  
  Purchase #6 $147.79   6.816%   $157.8634   $157.86   $157.85   $157.90  
  Purchase #7 $238.35   4.484%   $249.0376   $249.04   $249.05   $249.00  
  7 purchase totals = $635.50       $675.9505   $675.95   $675.95   $675.90