Long vowels and accents in perfect subjective

Nearing the end of my second reading of Familia Romana, I am getting ready to make study cards for the perfect subjunctive, but I have run into a dilemma. Some authorities show a long vowel in some syllables, and others do not. Thus

Bennett's New Latin Grammar: amāverim, I may have loved; amāverīs, you may have loved; amāverit, he may have loved; amāverīmus, we may have loved; amāverītis, you may have loved; amāverint, they may have loved;

and Wheelock's...

Long vowels and accents in perfect subjective