Parenthesis are "reserved sub-delims" as defined by the RFC 3986. That means that the character may have special meaning in certain parts of the URL. Here is what the RFC says about how they should be treated:
URI producing applications should percent-encode data octets that
correspond to characters in the reserved set unless these characters
are specifically allowed by the URI scheme to represent data in that
component. If a reserved character is found in a URI component and
no delimiting role is known for that character, then it must be
interpreted as representing the data octet corresponding to that
character's encoding in US-ASCII.
You are using the parenthesis in the path section of the URL. Parenthesis do not have special meaning in that section of the URL. The only restrictions on the path appear to be:
The path is terminated by the first question mark ("?") or number sign ("#") character, or by the end of the URI.
Therefore you may use parenthesis in the path of the URL, but you could encode them with
%29. As w3d says in the comments, software that uses heuristics to pick URLs out of text may not recognize the full URL when it contains unescaped parenthesis.