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 $60.35   10.684%   $66.7978   $66.80   $66.80   $66.80  
  Purchase #2 $155.80   5.236%   $163.9577   $163.96   $163.95   $164.00  
  Purchase #3 $185.70   3.775%   $192.7102   $192.71   $192.70   $192.70  
  Purchase #4 $24.64   13.223%   $27.8981   $27.90   $27.90   $27.90  
  Purchase #5 $69.67   12.151%   $78.1356   $78.14   $78.15   $78.10  
  Purchase #6 $137.39   9.695%   $150.71   $150.71   $150.70   $150.70  
  Purchase #7 $14.77   11.359%   $16.4477   $16.45   $16.45   $16.40  
  7 purchase totals = $648.32       $696.6571   $696.67   $696.65   $696.60