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 $139.25   12.432%   $156.5616   $156.56   $156.55   $156.60  
  Purchase #2 $198.30   10.770%   $219.6569   $219.66   $219.65   $219.70  
  Purchase #3 $208.30   11.200%   $231.6296   $231.63   $231.65   $231.60  
  Purchase #4 $250.89   7.226%   $269.0193   $269.02   $269.00   $269.00  
  Purchase #5 $5.27   13.527%   $5.9829   $5.98   $6.00   $6.00  
  Purchase #6 $127.69   9.182%   $139.4145   $139.41   $139.40   $139.40  
  Purchase #7 $208.63   6.758%   $222.7292   $222.73   $222.75   $222.70  
  7 purchase totals = $1,138.33       $1,244.9940   $1,244.99   $1,245.00   $1,245.00