Large sequence cache in Oracle
No it wont backfire. If there is confusion, you can read the manual to see the effect of caching. It means first time you select from sequence, it will cache next 999999 values in sga. It also means, for next 999999 sequence nextval calls you will not be updating seq$ table.

The downside of this is, if your instance or db crashes, those cached (and unused) values are lost. So if you need to have sequential values with no sequence loss, this will not be helpful. Along with caching, you perhaps want to use ORDER option to ensure sequences will be distributed in a ORDERED manner.