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

// Is this the only solution?
if (renameAll)
{
    DataGridViewRowCollection rows = dataGridView1.Rows;
}
else
{
    DataGridViewRowCollection rows;
    foreach (DataGridViewRow row in dataGridView1.SelectedRows)
    {
        rows.Add(row);
    }
}