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 $90.61   8.639%   $98.4378   $98.44   $98.45   $98.40  
  Purchase #2 $74.79   4.721%   $78.3208   $78.32   $78.30   $78.30  
  Purchase #3 $69.51   7.464%   $74.6982   $74.70   $74.70   $74.70  
  Purchase #4 $72.90   9.207%   $79.6119   $79.61   $79.60   $79.60  
  Purchase #5 $176.72   5.807%   $186.9821   $186.98   $187.00   $187.00  
  Purchase #6 $136.76   12.116%   $153.3298   $153.33   $153.35   $153.30  
  Purchase #7 $226.35   3.814%   $234.983   $234.98   $235.00   $235.00  
  7 purchase totals = $847.64       $906.3636   $906.36   $906.40   $906.30