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Thanks John Tony & Jorge
Unfortunately, I mostly work with smallish species, invariably 5mm or less and often less than 2mm in length:- the genit are correspondingly small and could easily get lost in the tube.
I do often mount up the genit prep for dried material (usually in euparal between 2X6mm coverslips inserted in card with a similar hole in it and then mounted on a pin) but doing this for a wet specimen and thus separating it from its genitalia is not an option.
There are of course many problems with any solid mounted approach but a major one for me is that the genit can't be rotated to see its 3-dimensional structure.
I think Tony's point about ease of examining dry material is a good one although a bit of practice, a fine brush and suitable watch glasses or similar, the difference diminishes greatly.
The potential to separate specimens from their labels is very real however and needs a considered & methodical approach.

I really don't think that there is a single and simple solution to what is the 'best' way to preserve Diptera. Each method has merits and demerits depending on objectives, personal preferences, facilities and resources. We just need to be aware of these advantages and limitations and be alive to possible ways to improve things.

A (slightly) related topic which perhaps should be a separate thread:-
I have recently been experimenting with collecting bulk samples into lactic acid which partially clears and fixes the specimens. Colour is changed of course but the partial clearing seems to not only make the genitalia more examinable, but also enables many other external features to be seen more clearly. Specimens can be transferred to alcohol or slide-mounted subsequently.
It might be a technique with potential for groups where morphology is more important than colour perhaps?
cheers
Adrian