I would say for Sindarin and Quenya technically either way can be employed. Tolkien referred to a 'general use' of the period of the tale, applicable to both S. and Q. [in a letter to Anthony D. Howlett, written in 1969], and himself provided imladrist in the Elvish letters [and Rivendell].
That said, in the Appendices of The Return of The King the general 'rule' appears to be that for a language in which many words end in a vowel, like Quenya, the vowel mark is placed above the preceding consonant (eliminating the need for a final carrier at least).
When writing Sindarin, for example, one can use a full mode where the vowels can be full letters or 'tengwar', as seen on the Door to Moria, or [generally speaking] a mode that employs tehtar, as in one of the King's Letters for example (published in The History of Middle-Earth).
For languages such as Sindarin, in which many words end in a consonant, the vowel mark was placed above the following consonant (when using a mode that employed tehtar of course).
Again I would stress that this seems to be the 'general rule' as noted in Appendix E, as Tolkien himself did not follow this in all examples.
Just to add, you could also leave out the vowel a when writing Quenya.
In Quenya in which a was very frequent, its vowel sign was often omitted altogether. Thus for calma ‘lamp’ clm could be written. This would naturally be read as calma, since cl was not in Quenya a possible initial combination, and m never occurred finally. A possible reading was calama, but no such word existed.
Wesome! Sorry, I meant awesome there