Best to get everything done at one time so you don't have to pay multiple charges for labor assuming you're getting it professionally installed.
For example, I went to Chicane Sport Tuning and they charge 3 hours of labor @ $70/hour for spring install. If I get the shocks and struts done at that same time with the springs, it's still 3 hours of labor, hence it's $210.
Let's say I got my H&R Super Sports and left my stock struts/shocks in. They'll probably last about 4 months with the extreme lowering H&R Super Sports give. Then I have to go back and get new shocks and struts installed for another 3 hours of labor @ $70/hour.
Better do it right the first time and be done with it. I got my springs, shocks/struts, caster/camber plates and alignment done all in the same day and have no trouble with my suspension/alignment at all.