// Check if we should rename ALL rows
DataGridViewRowCollection rows;
if (renameAll)
{
    rows = dataGridView1.Rows;
}
else
{
    foreach (DataGridViewRow row in dataGridView1.SelectedRows)
    {
        rows.Add(row);
    }
}

// Check if we should rename ALL rows
DataGridViewRowCollection rows;
if (renameAll)
{
    rows = dataGridView1.Rows;
}
else
{
    rows.Add(row);// Will not cause an error other than "row" not existing

    foreach (DataGridViewRow row in dataGridView1.SelectedRows)
    {
        rows.Add(row);
    }
}