Is mRNA isolation necessary for sensitive RT-PCR?

Usually not. Since RT-PCR is extremely sensitive, as little as 10–200 ng total RNA is sufficient for each 25–50 µl reaction mix, depending on the RT system. For abundant mRNA species, it is possible to use even less than 10 ng total RNA. For rare mRNA species, use a sequence-specific primer in the RT reaction to increase sensitivity. RNA content in various cells and tissues can be found here.