Quadrature Amplitude Modulation: I and Q Same
It doesn't really affect anything at all. If you modulate a signal on the I channel only, then it would look something like:

$$
x_1(t) = m(t) \cos(2\pi f_c t)
$$

If it were on the Q channel only, then it would look like:

$$
x_2(t) = m(t) \sin(2\pi f_c t)
$$

$x_1(t)$ and $x_2(t)$ only differ by a phase shift of 90 degrees on the carrier. Likewise, if you were to split the signal evenly in power across the I and Q channels, it would look like:

$$
x_3(t) = \frac{1}{\sqrt{2}} m(t) \cos(2\pi f_c t) + \frac{1}{\sqrt{2}} m(t) \sin(2\pi f_c t)
$$

Again, in the complex baseband form that is often used for analyzing signals, this corresponds to just a phase shift of 45 degrees from the original signal $x_1(t)$. In most systems, you will have a random phase shift between the transmitter and receiver anyway; this should not have any effect on the performance of your receiver.