But when she cooed the sweet refrain "Let's wait awhile/ Before we go too far", you had to laugh. It was a bit late to worry about exercising restraint - by this point we had already seen a clown open an enormous storybook, on to which was projected a psychedelic light show that was either a homage to Pink Floyd, or an attempt to hypnotise the audience into shelling out for more merchandise; then some fireworks went off, before Janet arrived on stage perched at the top of a step-ladder dressed in the style of Annie Hall, and proceeded to dance among a platoon of funeral directors. I suppose this sort of thing is considered normal in her family.
Part of the show's problem was that it transpired to be very normal indeed.
In the first section, which culminated in a medley welding together whip- cracking funk work-outs like "Nasty" and "What Have You Done For Me Lately?", Janet proved that she has more bite and style in one of her heavily-mascara'd eyelashes than her more dotty brother has in his entire bleached-out body.
She did a great impression of someone tough when she stood in the spotlight and grimaced at the audience - or as good an impression as somebody whose personal gurus and irrigators are hovering in the wings could ever be expected to do. And her dancers were outstanding with their old-school breakdancing and their wearing of awful tracksuits.
But all this Walthamstow Market-style braggadocio was in vain because the music was bad, and the sets were worse.
Janet saved her most bewitching songs until last - the lovely "Got `Til It's Gone", which hinges on a burst of Joni Mitchell (from "Big Yellow Taxi") and reclaims the honourable art of sampling from bad rappers; and the slinky "Together Again", which is the aural equivalent of the best massage you've ever had.
Those numbers sparkled despite the cluttered arrangements, but on everything else the over-sized band sounded as though they were playing their instruments while tumbling down a wrought-iron spiral staircase.
What you heard was blissful compared to what you saw. Apparently, the art director and stage designer have gone into hiding after fearing revenge attacks for their part in constructing the candy-coloured nursery which was unveiled in act two. A rubbery-faced crescent moon surveyed the dancers, who were done up like jesters, puppets and sticks of Blackpool rock; it was like being trapped in the doodle-pad of a lonely 9-year- old girl-guide.
And it sat uncomfortably with later attempts to drag the show beyond its nursery rhyme mentality. On "Rope Burn", Janet strapped a gangly male fan to a chair and performed a pole-dance for him, while the video screens offered us close-ups of his dribbling mouth which went some way toward extinguishing any stirrings you may have felt.
More incongruous was "What About", which married balletic choreography to scenes of domestic violence, a combination which would bring an unhappy tang to the most adventurous palate. And when things got a bit drab, Janet resorted to flashing her bra. That got squeals of delight from the audience, but then this lot went gaga over the drum solos, so that's not saying much.