My mom's cousin does this when she's wearing her micro-braids. I wouldn't really suggest it, but I can sorta see how why people do it since your edges and nape (if wearing your hair up) are more visible to other people. If you do this, you have to make sure that you space your relaxers out right. Ok, how about this.
2 months from now is December; you relax your edges in December. 2 months from December is February; therefore you can relax your entire head in February (including edges). 2 months from February is April; you can relax edges (only) in April. 2 months from April is June, and you can relax your entire head (including edges) in June. With this "system," you'd be relaxing your edges/nape every 2 months, but the rest of your head is only relaxed every 4 months....I hope this makes sense.
Like I said, this could possibly work if you space out your relaxers and have a set time for them.