Which altitudes are safest for VFR?
The altitude you fly at should be determined by the following:


Where you are safest.  Crossing 5500 foot mountains at 6500 simply is not a good safe practice.  Your altitude should be higher at night, if for nothing else, to give you more time to diagnose a problem or pick a suitable landing spot.  When my primary students fly the hills around here, I insist that their final cruise be above 3000 AGL.  That keeps them out of 2000 foot towers (most are just 1500 feet), and gives them time to pick a landing spot, and gives them altitude for getting radio assistance from FSS or ATC.  Higher is better, but I tend to not encourage 10,000 foot level flights for student pilots because there are other risks.  You should consider all the risks associated with altitude.  Lumbering along at 11,000 feet may not be the least risk option.
Your altitude selection should help you in navigating.  Being up higher lets you pick landmarks, see more VORs, reach more ATC and FSS facilities.  Generally, my students do not use GPS (as student pilots), so if they get lost, they have to descend and read water towers and things like that to find themselves.  That doesn't happen often.
Your altitude should keep you in good weather.  If you are a VFR pilot, you will probably not want to make your trip on top, with a 1000 foot ceiling below you. Again one needs to balance risk.  Being at an altitude which keeps you below the scattered layer, keeps visibility good, and can make a trip more enjoyable.  But if Nervous Nancy is taking her first plane trip, and is predisposed to motion sickness or is simply nervous about bumps, then you want to pick an altitude that will be a bit smoother.


The whole exercise of picking the right altitude, the right route and even the right day is part of a big optimization problem, where you are looking at safety, enjoyment, economics and other factors.  There is no magic altitude, no perfect route.  What is good 80% of the time may be not so good 20% of the time, which is why ADM (Aeronautical Decision Making) is emphasized in training and recurrent training.  Having good visibility, nice tailwinds, a smooth ride and plenty of altitude over the terrain are always nice.

Oh, one exercise you might try...I tend to do this with flight review students...we do airwork at altitude over the hills or low mountains about 25 to 30 miles from the airport.  I pull power and they dive for the nearest pasture, which has down slopes which exceed the descent profile of the aircraft with power off, the gear and flaps down, and heavy slipping.  Or select a field which has an up slope which is extreme enough so that a landing is impossible.  Then I suggest that they try the home airport.  "But it's 20 miles away."  or maybe 25, and they might have a good tailwind at 5000 feet above the airport.  And they start the glide.  On the way we pick alternates, including flat grass strips, along the way.  Almost 100% of the time, the lightly loaded aircraft makes the airport, and they are amazed that they were able to travel so far without power.  And the take-aways are many, which include that having a couple of thousand feet below you really opens up your options.  And to not pick the first field you see.  With altitude, there may be others as you get to more favorable terrain.

I have managed to fly many decades, many hours and never so much as scratched paint on an airplane.  However, before every flight and during every decision, I ask myself what an objective, after the fact, review board would say.