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 $238.86   12.798%   $269.4293   $269.43   $269.45   $269.40  
  Purchase #2 $120.57   11.135%   $133.9955   $134.00   $134.00   $134.00  
  Purchase #3 $125.99   10.186%   $138.8233   $138.82   $138.80   $138.80  
  Purchase #4 $53.95   12.756%   $60.8319   $60.83   $60.85   $60.80  
  Purchase #5 $29.29   3.780%   $30.3972   $30.40   $30.40   $30.40  
  Purchase #6 $206.81   4.689%   $216.5073   $216.51   $216.50   $216.50  
  Purchase #7 $128.64   9.369%   $140.6923   $140.69   $140.70   $140.70  
  7 purchase totals = $904.11       $990.6768   $990.68   $990.70   $990.60