If this form
is being presented as part of an audit
it is best to present the additional evidence prior to signing or to continue discussion of the issues by addressing the agent's manager.
Form 4549 proposes a determination that you owe more taxes. This is the auditor's proposal and may be revised if you present further evidence to the auditor or represent the evidence to their manager (especially if the auditor does not seem to be incorporating all of the evidence).
If you do not have the evidence ready to present then not signing will likely just lead to a Notice of Deficiency based on the auditor's proposal as seen on the 4549.
Of course appeal is possible later; but now is the time to ensure the record has all of the evidence and issues raised; as you will not often be able to have an appeals officer consider anything that was not presented to the auditor.
See, for example, the article at http://www.massachusettstaxalert.com/2014/10/new-irs
"Under the new AJAC Project, any investigatory or exploratory efforts should be performed by Examinations before the case is assigned to the Appeals officer. As a matter of policy, IRS Appeals officers “will not raise new issues” and, generally, “will not return
cases to Examination for further development.”"
Please ask if you need more information or clarification to understand the need to have all the issues raised during examination and not later in appeals.