If your theme is not responsive, then by all means: yes. Say, you have 900px wide content area to work with. You'd like 3 thumbnails at 300*200, with no spacing. Even if your photos have different aspect ratios, you can force-crop them using the Thumbnail aspect ratio setting. You'd enter 3:2 or 300:200 or 1.5, as they would do the same thing. Also, you'd just set Target row height to 200 with Row height max deviation set to 0.
However, your site is most likely responsive. In the responsive world, there is no such thing as x images per row. Why? Imagine that (in text) the optimal number of characters per line is 50-60. Would it still be comfortable to read on a small mobile screen? Indeed, it would be too small to read. So, if 5-6 images fit comfortably in a row on desktop, would they be fine on a mobile screen? Again, no. So the x images per row idea is flawed at this point, unfortunately. Let Justified Image Grid do its job and determine that 1-2 thumbnails per row is the most optimal on mobile devices. Still, it'll all depend your Target row height values. In practice, you can influence it to fit only 1 thumbnail per row, or force it to squish in many small thumbnails.
The fixed thumbnail size has a conceptual problem. It could be guaranteed on a mobile device, but no other image at that size would fit next to the first one. In the end, it would be impossible to have a justified layout that way. Therefore, an aspect ratio value is introduced, which can be fixed or inherited naturally from the original picture.