You can choose any number between 0 and 1024. It will not affect your mining performance in any way ! Currently 0 would be good to balance out all the 512 votes.
If you and everyone else start voting low the block reward will get lower after ~3600 blocks/votes(5 days) are in. You can see the current blockrewards and votes for next blockreward in the bottom left corner of your wallet.
As history has shown us block halving/lowering rewards is always driving prices up. It's way better to get fewer coins but have their prices steadily growing than having more coins and to discover that they have lost a proportion of their value before you sell them