Is it safe to use SUM() without ISNULL()

Yes its safe . You can use Sum without handling NULL Value. You can also check that.

You can use like that also.

ISNULL(SUM(COL1),0).

Returns the sum of all the values, or only the DISTINCT values, in the expression. SUM can be used with numeric columns only. Null values are ignored.

For Reference : https://msdn.microsoft.com/en-IN/library/ms187810.aspx