I don't see this as a problem, providing you introduce the tissue immediately in a lysis buffer containing agents to inhibit RNases and you proceed quickly with RNA isolation using well established methods such as Qiagen's RNeasy or miRNeasy kits.  Your saving grace is the immediate freezing in liquid nitrogen, followed by storage at -80 degrees.  However, for long term storage, the tissue should have ideally been stored in RNAlater or equivalent, or stored in liquid nitrogen.

Yes you can do it but, I completely agree with Amadeo.

But in my opinion, if you should extract RNA I guess is better to perform tissue homogenization directly into the proper isolation solution. For example if you're going to extract RNA using Trizol/Tri-reagent/RNAzol is better to homogenize directly into this solution. Dissection and tissue lysis drive the release of endogenous RNAses and these enzymes are active in phosphate-buffered solution. Different is the situation when you're performing homogenization into a proper solution such as Trizol or other reagents containing RNAses inhibitors. The activity of enzymes in this case is disrupted and also the extraction of nucleic acid is increased. For example we use to dissect mosquitoes in D-PBS but we transfer the tissues immediately into a RNAse free vial with RNAlater solution on ice. We add Trizol immedately after finished the dissections into a fume hood and we store the samples at -20°C for short term storages or we process the samples immediately by homogenization into Trizol solution. You can also store the samples already homogenized in Trizol at -20 for short term storages or -80 for long term storages or freeze samples unhomogenized placed into RNAlater solution, and homogenize the samples directly adding Trizol prior to perform extraction.  

Also if you're going to extract RNAs using a dedicated kit is better to perform the lysis/homogenization directly into the lysis/homogenization buffer provided with your kit.

Assuming you already homogenize your tissue with phosphate buffer and you want to extract RNA from it. Can you first centrifuge and discard the supernatant before adding RNA lysis buffer to sedimented tissue or you just add RNA lysis buffer to the already homogenize tissue.