The processor model I use is i7 core 2675 QM in mac pro late 2011 model. On the details site  I cannot find the voltage specification for RAM! IS 1.35V better or 1.5V for the original design? How much of a difference will it make if i use the 'wrong one'?
Thanks in advance!
 URL = https://ark.intel.com/products/53470/Intel-Core-i7-2675QM-Processor-6M-Cache-up-to-3_10-GHz Intel® Core™ i7-2675QM Processor (6M Cache, up to 3.10 GHz) Product Specifications
You should use whatever voltage memory is stated in the specifications for your system. If it allows 1.35V memory usage, this is better. I believe, however, that most 2nd generation processors were paired with only 1.5V memory.
In the specification that I cited in the question(from intel), there is no mention of voltage. I too think it should be 1.5V but wanted a reliable answer. And also what happens if I use 1.35 V instead, assuming 1.5 is the specification?
Crucial and OWC RAM recommendations for the model list both kinds of voltages as doable!