Some vendors recommend that when using their primers for house keeping miRNA genes along with universal primer and SYBR Green master mix, it is not necessary to treat RNA preparation with DNAse to remove gDNA contamination, prior to making cDNA. In our hands, we still get amplification albeit at low level when we use total RNA for cDNA synthesis without RT, indicating the presence of gDNA in the prep.
If your contaminating gDNA signal is at least 6 cycles or more away from your RT+ reactions, then the addition to signal by lingering gDNA becomes mathematically insignificant, for the most part.
Sometimes DNase enzymes are contaminated with low level of RNase enzymes -and is likely the reason vendors discourage the overt use of DNase (for fear of degrading RNAs of ~22 nt length too voraciously and inadvertently). Notice how certain vendors warn against prolonged DNase treatments on account of this in general.
I agree with your explanation of contaminating RNases. However, it is important to get rid of gDNA as much as possible to make sure that the amplification is not due to gDNA contamination. We see a significant difference in the amplification with and without DNase treatment.
I personally always use excessive DNase treatment of RNA to ensure gDNA is a non-issue. The company may be in error in their suggestions on this or, the company assumes the primers span a genomic intron:exon junction associated with miRNA, therefore primers shouldn't amplify the gDNA...
MicroRNA genes are shorter ~20-22 nt and most are in the introns and transcribed with the host genes. Given the size of miRNAs, it is difficult to design specific primers. I found out that DNase treatment has dramatic has effects on the amplification of some of the popular house keeping miRNA genes that the company suggests that it is not necessary to treat the miRNA prep with DNase for qPCR amplification.
As part of the microRNA quality control study (miRQC), we tested the specificity of 12 miRNA gene expression platforms (sequencing, hybridization, PCR) with respect to genomic DNA. Data is not included in the final manuscript (under consideration), but there was generally no or only minimal impact of contamination DNA. However, it is good practice to have your RNA DNase treated.
The miRQC paper is out now in Nature Methods.
http://www.biogazelle.com/knowledge-center/publications#node-784
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