It depends whether you use air conditioning. if yes, i use 1.0 tog, cos after midnight, the body gets colder & it'll be safer to use the 1 tog. my building is currently doing external walls renovation & they take down all the air-cons, and i'm using 0.5 for my 2 yo.
the best thing to do is, follow the thermometer that comes with the grobag, and follow recommendation for the rm temp in baby's rm.
(i have 2 of each tog for throughout the yr use. if you're planning to use grobag until your baby is beyond 12 mths, i think it's worth it to invest in all 3 thickness.)