Had this at my old workplace when I worked in IT for an agency. Whilst the workload usually justified some extra working late to meet demand, it was too far ingrained in the 'culture' of the company. The directors did it, so everyone else was expected to and you would feel bad for leaving on time. It was however stipulated on our contracts that we would have to work extra hours to meet demand without extra compensation, so covered by contract.
I stuck around for a while and tried to tolerate the stress but eventually it started to affect not only my work but personal relationships too. I left on good terms, but vowed to be very wary of any company that requires full-time staff to work extra to meet demand without recompense, always read the contract I guess.
Definitely keep your head down and crack on, work some lates if you have to, but make sure you have a solid exit plan / excuse lined up for when you can't/won't. The company will be expecting you to move heaven and earth in your probationary period to prove yourself, after all, that's the kind of people these businesses are looking to keep on long term.