I'm worse than that. I plan around astrological signs. I've avoided the three signs that are the least compatible with DH and I. Okay, maybe that's an exaggeration. I don't try nearly as hard on those months, and I err on the side of caution in trying for my little girl (cut off is 3 1/2 days before o, instead of 2 1/2) to make sure that if I have to put up with a child from a particularly stubborn sign, at least I'll be sure
she's a girl! Besides, my careful planning may end up meaning very little, since DS was born 3 weeks early and ended up being a Cancer, instead of a Leo. Didn't matter between those, though. Geminis and Aries are fine with either
So, to answer you question, no, I don't think avoiding TTC to space b-days apart is bad. I also think it's fine to avoid the holiday season (a lot of people I know with December b-days hate the "this is both your birthday and Xmas present" thing) or trying to make sure your pregnancy isn't at a difficult point at a really uncomfortable season/event. I know women who planned to give birth a couple of months after a move so that they didn't have to juggle a newborn and packing. I have a friend who planned to give birth in fall/winter to maximize the amount of time she'd have before fitting into a bathing suit again!
Ultimately, it's up to you to decide if it's worth giving up a month of TTC for convenience. It's taken me so long that I'm pretty much trying hard every month now and throwing my concerns about compatible signs to the wind! Of course I say that now, because if I got preggo this month I'd have an Aquarius, and next month would be a Pisces. I'd be happy with either of those!
Good luck making your decision!