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 $209.79   10.374%   $231.5536   $231.55   $231.55   $231.60  
  Purchase #2 $185.74   7.222%   $199.1541   $199.15   $199.15   $199.20  
  Purchase #3 $138.58   4.728%   $145.1321   $145.13   $145.15   $145.10  
  Purchase #4 $9.27   3.516%   $9.5959   $9.60   $9.60   $9.60  
  Purchase #5 $225.39   6.108%   $239.1568   $239.16   $239.15   $239.20  
  Purchase #6 $10.56   3.522%   $10.9319   $10.93   $10.95   $10.90  
  Purchase #7 $81.60   8.809%   $88.7881   $88.79   $88.80   $88.80  
  7 purchase totals = $860.93       $924.3125   $924.31   $924.35   $924.40