I guess it is a personal preference. I know that for me, I have always wanted a baby RIGHT NOW, so I never wanted to wait.
You really need to consider the end result when deciding how long to wait. Do you want 3 kids by the time you are 41? Then you need to be pregnant already. Do you want 2 kids that are 2 years apart? Then if you got pregnant now, you would deliver when you are 39, then you would ttc when you are 40 or 41 to have baby #2, at which time it might get more difficult.
If you only want one child, then I think you could wait it out for 6 months, but I wouldn't wait too much longer than that. Sure, there are lots of women that get pregnant easily when in their late 30s, but there are plenty that struggle too. You don't know which boat you will be in, so I wouldn't waste too much time. It's normal to take up to a year to conceive naturally at 38. So, that just pushes back the age at which you can start to ttc for #2.
I would be very proactive and use opks, temp, and research fertilityfriend.com for charting info until you decide you are ready for intervention.
Good luck. I hope you get your bfp soon and don't even have to worry about the doctors.