During the eligibility period, I worked for only 4 businesses, so had experience certificates from all 4 and had listed them individually with start and end dates of the contract period.
I do not know if they were contacted or not. I wasn't working at any of those corporations after my application, so had / have no way of finding out.
I would recommend getting some supplementary evidence in place as well like rent receipts / contracts, mortgage papers or any other documents that prove your physical presence. You don't have to send it, but keep it ready in case asked.
Utility / Credit card bills are usually unhelpful, but regular transfers from your employers to your account could certainly help. I'm not trying to scare you, but I had a pretty rough time proving my physical presence here, because I had a history of travel, but it doesn't hurt to be better prepared.
Good luck.