Is it proper to leave amp on all the time?

Hi, I recently started to try to build a system that for years that I thought was way out of of my league. Going back 35 years ago it just seemed when everybody was so interested in speakers that could blow you off the planet, I walked into a audiophile high fi store & my outlook on stereo forever changed. (Source Linn LP12 turntable to Speakers), better to have a solid front end, working through mid section (Amp, Pre-Amp)and finally speakers. Sub-Par Speakers sound better than good speakers without good source. Anyway enough of that, this is a little bit of an introduction to myself!!! I recently bought a used Rotel DVR 995 $50.00, a used pair Vandersteens 2Cs $450.00,& A brand new NAD 326BEE Integrated Amplifier $450.00 After all of that, my question is that I was told by my Audio Dealer to just leave my amp on all the time, and if that is true, why? Seems like it would shorten lifespan of unit. Also I forgot to mention that along with the Rotel & the Vandersteens, I got Silver Sonic Q10 High Resolution Speaker Cables, Tributaries Audio OFHC Copper Cable Interconnects from the same person that I bought the Rotel & Vandersteens. I like all kinds of music for the most part!!! My dealer said he would give a 100% back on NAD if I was to return it within 2-3 months to upgrade. I was thinking that I would trade for the NAD375BEE which would be the most cost effective, before I went & started on the CD player which I could do the same as the NAD326 to buy some time to save for a nicer CD player. Then Cables, Power Supplies, Speakers etc. What is a good plan to get better sound each time I add something? I do like the idea of a warranty & the expertise of the local dealer who comes highly reccommended we'll see!!! Novice looking for guidance!!!