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 $83.85   7.205%   $89.8914   $89.89   $89.90   $89.90  
  Purchase #2 $122.83   9.787%   $134.8514   $134.85   $134.85   $134.90  
  Purchase #3 $22.71   3.806%   $23.5743   $23.57   $23.55   $23.60  
  Purchase #4 $228.61   8.152%   $247.2463   $247.25   $247.25   $247.20  
  Purchase #5 $49.97   6.497%   $53.2166   $53.22   $53.20   $53.20  
  Purchase #6 $189.29   13.775%   $215.3647   $215.36   $215.35   $215.40  
  Purchase #7 $166.99   5.396%   $176.0008   $176.00   $176.00   $176.00  
  7 purchase totals = $864.25       $940.1455   $940.14   $940.10   $940.20