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 $136.62   10.178%   $150.5252   $150.53   $150.55   $150.50  
  Purchase #2 $24.87   9.512%   $27.2356   $27.24   $27.25   $27.20  
  Purchase #3 $122.67   4.678%   $128.4085   $128.41   $128.40   $128.40  
  Purchase #4 $239.36   9.658%   $262.4774   $262.48   $262.50   $262.50  
  Purchase #5 $39.77   9.620%   $43.5959   $43.60   $43.60   $43.60  
  Purchase #6 $236.41   7.826%   $254.9114   $254.91   $254.90   $254.90  
  Purchase #7 $13.75   7.851%   $14.8295   $14.83   $14.85   $14.80  
  7 purchase totals = $813.45       $881.9835   $882.00   $882.05   $881.90