External recruiters are normally compensated by a percentage of your first year's salary, so disclosing a salary requirement to them is actually usually less likely to decrease your earnings than disclosing it to an internal recruiter. The internal recruiter is never going to offer you more than your salary requirement unless you get a better offer, whereas the external recruiter will be quite happy to hint that the hiring company might be able to budget for more if they don't think you're less likely to be hired at a higher rate.

Deal with external recruiters if they're offering you jobs with higher prospective salaries than you're able to find from your own enquiries, and don't if they don't. External recruiters make huge amount of money placing people in positions they could have easily applied to themselves, but they also find out about positions you're unlikely to find simply by looking.