You'll be fine with 8gb of RAM specifically with a 13" MBP for a couple reasons.
Knowing the machine has an iGPU your workload would need to be specific enough that you'd know if you needed more RAM. Example, allocating RAM to VM's....
While not always the case the vast majority of task that use a lot of RAM and need physical RAM (not compressed and not swap) are graphic intensive. So your bottleneck will be the iGPU for those task.
Since I have a 13" MBP i5 I performed a little demo. All I did was open DaVinci Resolve, opened a 4k@30hz video taken on my iPhone 6S and just started adding effects.
The entire time this is as bad as memory pressure got....
Not good but reasonable considering the task I was asking of the laptop. The poor CPU/GPU though....
The line at the far right is max temp and CPU cores 1-3 hit 100c and tjunction throttled performance. Even using proxies at 1/4 resolution was getting 20-30 fps and adding effects might as well been paused.
Now a more reasonable request would be iMovie and GIMP for image editing and the 13" MBP handles like a champ especially considering Safari, Mail, Messages, etc were also running in the background. Now this isn't fair since DaVinci does more it requires more but this is while using every option available in iMovie at the same 4k@30hz video.
Preview playback was smooth, never dropped off of 30fps.
The point I'm alluding to is 8gb will be fine for the 13" MBP because without a specific workload you'll likely be bounced off other performance bottlenecks. Getting 16gb of RAM
could have little to no effect on system performance.
All that said, if 16gb is available and you can comfortably afford it definitely go for it. It won't hurt anything and it if you ever get an eGPU and your workload changes 8gb RAM could become a real problem.
Although I currently don't have one I try to keep my laptops on the less expensive side and use a more powerful desktop.