How much bigger can slacks be before altering them to fit becomes too arduous for an average tailor? Is it foolhardy to even think of buying a size 34 when I'm a 30?
First, measure the waist to be sure they will really need to be taken in 4". Depending on the cut, where they are supposed to ride (waist? hip? midthigh?), the manufacturer's labeling tendencies and the phase of the moon, a 34" may mean anything. I have some 34s that barely fit me and some 30s that are comfortable but not even snug. Altering is a question of balance. Whether the back pockets will appear too close together and the line of the crease up the back of the leg will be thrown off is the major concern. Also, since I assume most average tailors will be altering only from the rear seam, the side pockets may end up too far back as well. Anyone can sew a new seat seam two inches in from the original and call it a 34"->30" transformation. The question is whether the pants will be presentable afterwards. My guess in general is no.