If your design totally uses positive edges it is not that important... you can use uneven buffers usually with no problem.
However, ...there is a small but here.... there is a small danger that very uneven edges, in very high frequencies close to the natural frequency of your buffer, can result in "attenuating"/muting your clock edge... but this is not something that you should usually worry about unless you are working in very high frequency...
Anyway, you shouldn't really use negative edge.. if you want the negative edge of a clock, invert it with a good buffer, make a good clock tree and use the positive edge