Nope. I have a 210j on which I run some tests and reinstall DSM frequently, and after I configure a static IP, next time I reformat it the Assistant tries again to propose a dynamic IP settings. Even after re-selecting static IP, it has no memory whatsoever of the old IP used.
When you select a specific IP, Synology Assistant is actually pinging that IP address (from the PC you are running SA). If it receives an answer, it means that some device is actively using that IP.
Please *do* test settings, don't just work by assumptions (like "IP a.b.c.d should be free").
DS-411 (DSM 4.2-3202) w/ 2x WD10EFRX + 1x HD154UI
DS-106j (DSM 3.0-1357), PATA-to-SATA adapter, 2.5" HM250HI