No, it is not necessary for you to make the estimated payments just because the vouchers printed with your return. If your income situation will be different for 2019 it is fine to ignore the estimated payment suggestion.
For the second part of your question, the idea of making the payments on a quarterly basis is so that you are paying as you go. Making only 2 payments instead of 4 would probably be ok, unless there was still and underpayment situation at the time of filing your tax return.
For more information, see the TurboTax article below.
https://ttlc.intuit.com/replies/3301872
