Advice plz............?

In sfm aaditya jain sir has given taken two alternatives for all assumptions in question....first alternative can be taken in exam,for this alternative marks won't be deducted in exam as per sir...and for second alternative,this alternative is preferred by c.a institute...so please tell me in some questions I like first alternative and in some questions I like second alternative....if I don't take institute prefereence then marks will be deducted and for all assumptions taken in first and second alternative,note is to be given in exam that we have taken this assumption? And. Also tell me upto how much decimal places we have to take in solution in sfm and can we do round off?