Model:

[HierarchyRoot]
public class TestEntity : Entity
{
    [Field, Key]
    public Guid Id { get; set; }

    [Field]
    public decimal Dec { get; set; }
}

Test:

new TestEntity
{
    Id = Guid.NewGuid(),
    Dec = 274486.4m
};

var fromQuery = Session.Current.Query.All<TestEntity>().Sum(x => Math.Round(x.Dec, 2));
var fromList = Session.Current.Query.All<TestEntity>().ToList().Sum(x => Math.Round(x.Dec, 2));

Assert.True(fromQuery == fromList, string.Format("{0} != {1}", fromQuery, fromList));

Exception:

274486.4000000000200000000 != 274486.40

DO 5.0.7

MSSQL 2012