Innerspring Mattress will sag, like all other mattresses but after prolonged usage and after a very long time. The average lifetime durability of an innerspring mattress is 15 years. You can rotate your mattress every 6-8 months to avoid sagging.
So, don't worry about sagging till then.
Sagging is one of the most common complaints from Inner Spring mattress owners. All types of mattresses will sag eventually. But when it does it will seriously affect your comfort level.
If you change the mattress position, then all sides will get exposed equally. This will delay sagging.