They don't check unless you look dodgy or are stopped for some other reasons, then you would need proof of funds ... but probably the least of problems if it comes to that. Having a visa ahead of time means you skip the first queue and head straight to the 2nd queue (immigration). Sometimes that can be worth an hour, but more typically it is 5 min. Getting a multi-entry visa can be difficult, however, esp. if you don't have a credit card.