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 $144.41   9.118%   $157.5773   $157.58   $157.60   $157.60  
  Purchase #2 $118.67   5.270%   $124.9239   $124.92   $124.90   $124.90  
  Purchase #3 $24.40   13.170%   $27.6135   $27.61   $27.60   $27.60  
  Purchase #4 $203.54   11.283%   $226.5054   $226.51   $226.50   $226.50  
  Purchase #5 $222.40   8.726%   $241.8066   $241.81   $241.80   $241.80  
  Purchase #6 $95.71   5.621%   $101.0899   $101.09   $101.10   $101.10  
  Purchase #7 $230.82   12.430%   $259.5109   $259.51   $259.50   $259.50  
  7 purchase totals = $1,039.95       $1,139.0275   $1,139.03   $1,139.00   $1,139.00