I use to have one of those old oil heaters setup during winter last year for the indoor grow and for any plants older than a month I don't think they need it at all. If my 1 month old chili plants can survive outside in -0.6 then any indoor plants should be fine. Specially switching over to lights on at night and turn them off during the day ( also cheaper on your power bill too) . Those HPS & MH bulbs are like huge heaters on their own.. It's that Summer that worries me 40+ days
Currently got the seedlings down stairs no heater on just a cheap arse heat mat and fluros and temps don't go below 18degrees, but the room is insulated.