What method are you using? If using the calibers, then yes, try to have the same person do it everytime. The gym I'm employed at offers caliber body fat testing to members. I cannot tell you how many times someone had their bodyfat measured by one trainer, then months later after losing some weight, goes back and has their body fat measured by a different trainer. Each time this happens the member is disappointed because the new trainer measured them HIGHER than before.
Now caliber body fat testing is not the most accurate, so don't so much rely on the numbers. BUT, if you go there one month and measure at 25% bodyfat, then go back next month and measure at 20% bodyfat. You may not actually be 20% bodyfat, but you know you lost some.
I don't like having my bodyfat tested, I find it to be a hassle. It certainly doesn't hurt to get it tested. You could try getting it done once a month or once every two months, however you see fit.