Yes EMI does drop with distance, the exact amount depends on the geometry of it's surroundings but could be as much as inverse square of the distance.
The rise and fall times you quote are quite slow and it the rise and fall that primarily produces the EMI so the chances of you having interference problems are very small. None of the signals around a 555 and most gate drivers are high impedance, it needs to be more than about 50K to be prone in that kind of circuit. With traces as short as 5mm you shouldn't worry.
Brian.