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 $20.28   5.243%   $21.3433   $21.34   $21.35   $21.30  
  Purchase #2 $36.98   12.171%   $41.4808   $41.48   $41.50   $41.50  
  Purchase #3 $22.65   7.278%   $24.2985   $24.30   $24.30   $24.30  
  Purchase #4 $10.85   6.456%   $11.5505   $11.55   $11.55   $11.60  
  Purchase #5 $151.33   13.223%   $171.3404   $171.34   $171.35   $171.30  
  Purchase #6 $15.99   10.554%   $17.6776   $17.68   $17.70   $17.70  
  Purchase #7 $159.32   3.801%   $165.3758   $165.38   $165.40   $165.40  
  7 purchase totals = $417.40       $453.0669   $453.07   $453.15   $453.10