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 $39.75   7.466%   $42.7177   $42.72   $42.70   $42.70  
  Purchase #2 $23.40   4.619%   $24.4808   $24.48   $24.50   $24.50  
  Purchase #3 $88.25   12.481%   $99.2645   $99.26   $99.25   $99.30  
  Purchase #4 $47.84   7.311%   $51.3376   $51.34   $51.35   $51.30  
  Purchase #5 $179.58   7.292%   $192.675   $192.68   $192.70   $192.70  
  Purchase #6 $127.64   4.458%   $133.3302   $133.33   $133.35   $133.30  
  Purchase #7 $91.44   9.271%   $99.9174   $99.92   $99.90   $99.90  
  7 purchase totals = $597.90       $643.7232   $643.73   $643.75   $643.70