hereで説明されているように、ボタンにスタイルを設定しないBTNS_SHOWTEXTと、ボタンのテキストには表示されませんが、マウスをボタンの上に置くと、テキストとともにツールチップが表示されます。  
このコード サンプル テキストがボタンに表示され、ツールチップが表示されない理由がわかりません。
#include <windows.h> 
#include <stdlib.h>
#include <CommCtrl.h>
#pragma comment(lib, "comctl32.lib")
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
HINSTANCE instance;
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
    instance = hInstance;
    WNDCLASSEX wcex; 
    wcex.cbSize = sizeof(WNDCLASSEX);
    wcex.style           = CS_HREDRAW | CS_VREDRAW; 
    wcex.lpfnWndProc    = WndProc; 
    wcex.cbClsExtra     = 0; 
    wcex.cbWndExtra     = 0;  
    wcex.hInstance      = hInstance;
    wcex.hIcon          = LoadIcon(hInstance, MAKEINTRESOURCE(IDI_APPLICATION));  
    wcex.hCursor        = LoadCursor(NULL, IDC_ARROW); 
    wcex.hbrBackground  = (HBRUSH)(COLOR_WINDOW+1); 
    wcex.lpszMenuName   = NULL; 
    wcex.lpszClassName  = L"Example"; 
    wcex.hIconSm        = LoadIcon(wcex.hInstance, MAKEINTRESOURCE(IDI_APPLICATION));
    RegisterClassEx(&wcex);
    HWND hWnd = CreateWindow(L"Example", L"", WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, CW_USEDEFAULT,
        500, 500, NULL, NULL, hInstance, NULL);
    // Initialize common controls.
    INITCOMMONCONTROLSEX icex;
    icex.dwSize = sizeof(INITCOMMONCONTROLSEX);
    icex.dwICC   = ICC_COOL_CLASSES | ICC_BAR_CLASSES;
    InitCommonControlsEx(&icex);
    // create toolbar
    HWND hWndToolbar = CreateWindowEx(0 , TOOLBARCLASSNAME, NULL, WS_CHILD | TBSTYLE_TOOLTIPS,
            0, 0, 0, 0, hWnd, (HMENU)0, instance, NULL);
    HIMAGELIST hImageList = ImageList_Create(16, 16, ILC_COLOR16 | ILC_MASK, 3, 0);
    SendMessage(hWndToolbar, TB_SETIMAGELIST, (WPARAM)0, (LPARAM)hImageList);
    SendMessage(hWndToolbar, TB_BUTTONSTRUCTSIZE, (WPARAM)sizeof(TBBUTTON), 0);
    TBBUTTON tbb[1] = 
    {
        { 0, 0, TBSTATE_ENABLED, TBSTYLE_BUTTON, {0}, 0, (INT_PTR)L"New" },
    };
    SendMessage(hWndToolbar, (UINT) TB_ADDBUTTONS, 1, (LPARAM)&tbb);
    SendMessage(hWndToolbar, TB_AUTOSIZE, 0, 0);
    ShowWindow(hWndToolbar , SW_SHOW);
    // show the main window
    ShowWindow(hWnd, nCmdShow);
    MSG msg;
    while (GetMessage(&msg, NULL, 0, 0))
    {
        TranslateMessage(&msg);  
        DispatchMessage(&msg); 
    }
    return (int) msg.wParam;
}
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    switch (message)
    {
        case WM_CREATE: 
            return 0;
        default:
            return DefWindowProc(hWnd, message, wParam, lParam);
    }
}