Job Function dropdown Forms

I'm looking to get some feedback and advice on dropdown forms. I have started segmenting our database and have found that as prior forms allowed people to enter their job title themselves, we have been left with alot of rubbish. 

Targetting people based on their job is something that has been repeatedly pointed out to me in the last few weeks so I am trying to make a new form for future use so that the database can be a bit cleaner for sales. 

I have selected all relevent job titles and created a form with these as a dropdown. In total there is about 20 titles included and I wanted to have the option for someone to select Other and then be prompted to enter their job title if it wasn't already listed? 

Also has anyone found the dropdown option for job titles is not beneficial for any reason? 

Thanks