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 $34.80   11.861%   $38.9276   $38.93   $38.95   $38.90  
  Purchase #2 $21.70   6.838%   $23.1838   $23.18   $23.20   $23.20  
  Purchase #3 $114.45   10.609%   $126.592   $126.59   $126.60   $126.60  
  Purchase #4 $190.91   9.820%   $209.6574   $209.66   $209.65   $209.70  
  Purchase #5 $84.65   10.834%   $93.821   $93.82   $93.80   $93.80  
  Purchase #6 $233.57   5.704%   $246.8928   $246.89   $246.90   $246.90  
  Purchase #7 $11.61   7.707%   $12.5048   $12.50   $12.50   $12.50  
  7 purchase totals = $691.69       $751.5794   $751.57   $751.60   $751.60