Hi jingsuet,
In my opinion if you are not fully confident & you are performing poorly in your practice tests, then you should postpone your test date, it is very essential to score 700 + Consistently (say 8-9 times out of 10) on the practice tests if you want to score 700+ on the real GMAT. Also you should take as many practice tests as you can, this will bolster your confidence level. To summarize what I believe is that:
"If you score 700+ in your mock tests consistently then you are relying on your efforts to help you score a 700+ on the GMAT, otherwise you rely on your luck"
Hope this is helpful.

dear jing,
Can you tell us more about your situation & prep level in detail?

...moved to strategy section.