It's preferable to capture from below but not mandatory.
You really should be on the localiser before descending on the glideslope.
In jets CDA (Constant Descent Approaches) are the most fuel efficient and in some places required for noise reasons, London Heathrow is an example where if you DONT achieve a low drag CDA your company will get a noise complaint.
Sometimes when I fly into LHR I end up on localiser intercept with the G/S bug a dot below me and sometimes a dot above me...I don't lose sleep over either but in a perfect world where I have juggled and guesstimated the ROD vs track nm perfectly I would be on intercept heading, LOC armed, with the GS just hovering above me. On final intercept and just before LOC comes alive I arm APP and the act of turning onto the LOC reduces my effective ROD a tad and I intercept both together...maybe LOC a few seconds before GS....and the thrust levers never left Flight Idle
