Ultimately, no dnsLookup() is not required for any web transaction, whoever it is you're posting to.
dnsLookup() resolves a web name into an IP address. If you already have the IP address you can skip the dnsLookup(), but you will have to ensure that you have the right Host: entry in the request header to ensure that the correct Virtual Host is selected on the remote server. This is most often put in by using the hostname portion of the URL, so if you just use an IP address in there you may end up requesting the wrong website. If you are manually crafting the HTTP request header yourself, then it won't be a problem as you can specify the correct hostname in the Host: header.