According to Mastering Bitcoin. Mining - Transaction Age, Fees, and Priority, miners sort transactions, using special parameter, called priority, which looks like

Priority = Sum (Value of input * Input Age) / Transaction Size

If the priority is greater than 57,600,000, it means that transaction is high priority and it will be written in the first 50 kb.

The question is: Is priority really necessary? I mean, if I'm a miner and I have e.g. 1000 unconfirmed transactions, is it ok to randomly choose 100 transactions and put them into the block just as it is? Without sorting by priority or fees?

Will it work out or I'll get reject message or something?