I don't know whether or not this is good advice, since I've never been married. But, with my outlook on life, I think the best thing to do is have them right away and "jump right" into it. Life is only as good as its hardest, worst moments, so you might as well deal with the obstacles and hard parts first.
On another note--this one I personally know about since I'm the oldest siblings--I think that it is better to have your first two (or three depending on how many children you plan to have) children VERY close to each other, with nothing larger than a two year age gap between them. The reason for this is because a lot of times parents have one oldest sibling, and then years later, they start popping out more children and expecting the one oldest child to be able to care for the rest of the siblings all alone.
At least if you have the first two or three kids very close to each other, the burden on them as the oldest children will be lighter in the future when they are babysitting for you, since they could share the work and take turns babysitting instead of one sibling having to do the whole thing.
My parents had me and my siblings all at different times. There is a 7 year age gap between all of us. I was born first, then 7 years later they had my brother, and then from there, 7 years later they had my baby sister. So there is a 7 year age difference between me and my brother, a 7 year age gap between my brother and sister, and a 14 year age gap between me and my sister. This is rather difficult because I have to care for all of them and any subsequent children my parents may decide to have. I personally would have preferred it if they had my brother within two years of having me, that way I'd have a closer friend and we could share the load of babysitting.