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 $134.39   3.745%   $139.4229   $139.42   $139.40   $139.40  
  Purchase #2 $119.66   10.726%   $132.4947   $132.49   $132.50   $132.50  
  Purchase #3 $176.93   13.285%   $200.4352   $200.44   $200.45   $200.40  
  Purchase #4 $198.97   8.517%   $215.9163   $215.92   $215.90   $215.90  
  Purchase #5 $184.82   8.261%   $200.088   $200.09   $200.10   $200.10  
  Purchase #6 $9.90   4.444%   $10.34   $10.34   $10.35   $10.30  
  Purchase #7 $146.43   9.129%   $159.7976   $159.80   $159.80   $159.80  
  7 purchase totals = $971.10       $1,058.4947   $1,058.50   $1,058.50   $1,058.40