in "synaptic" ? .. synaptic doesn't give "levels", only the updater (shield by the clock) does that.
If you're talking about Peppermint, then YES install them (it's just a new kernel)
Peppermint use the updater from Mint ..
Mint by default have set "levels" 4 & 5 to not install by default (unticked)
Peppermint on the other hand follows the Ubuntu update policy .. so ALL Levels should be set to install by default
If on the other hand you're using Mint .. it's your call whether to follow their policy of not updating kernels .. personally I think it's a stupid policy, but your call.