To "R" is human; to flip, divine

There are three kinds of "R"

American "R" (tongue moves toward, but does not touch gum ridge - can impact vowel)
Flipped "R" (tongue actually quickly touches the gum ridge once, and only once - no impact on vowel)
Rolled or trilled "R" (generally not used, except in especially dramatic context, or need to project)

Rules for Sounding or Omitting "R"
1. Never sing "R" before a consonant sound (no exceptions) applies within words, and in adjoining words.
2. Do not sing "R" before a pause (*one exception) Pause can be a notated rest, or articulation.
3. Always sing "R" before a vowel sound (no exceptions) applies within words, and in adjoining words.
(*When a diphthong dear or triphthong fire is followed by a pause on a high note, it is more effect to use flipped "R")

Which "R" to sing before a vowel?
Use flipped "R" between vowels.
Use American "R" in "" true and "dr" dream combinations (except on high notes)
For Gilbert & Sullivan choruses use flipped "R" most of the rest of the time (exceptions as noted in rehearsal)
This last is at variance with Madeleine Marshall's prescription of Art Song rules for G&S, but is born out by the special needs of chorus to project (and by listening to historical recordings)