Consequences of not following reflow profile
If you don't follow the reflow profile properly, there are a few things that can happen


If your board contains large copper pours, and are not warmed up properly, during reflow, your components can tombstone which is when a resistor or something will literally stand up and then is soldered in place like that.  This can be a pain to fix depending on where the component is.
If you heat up too quickly, you can thermally shock your components that are already placed on the board.  While there have been measures taken to reduce this during the design of chips, it is still a problem if procedures aren't followed properly.  Thermally shocked components may not function as expected until they are brought to regular conditions although sometimes they will flat-out break.
Another more obvious problem is that some of your components might not be soldered.  Its true that solder paste is metallic although solder paste alone is not a great connection.  If you don't reflow the board properly, some components might just not get soldered and will eventually fall off or cause other problems.


With those said, if your board is not the most dense board, like it doesn't have a bunch of decoupling capacitors underneath a processor, I wouldn't worry too much about it.  You can easily observe whether something is soldered.  It would still be a good idea to get a preheater since the copper pour issue is kind of annoying (You can literally watch it happen while using hot-air rework)

Since you do have a double sided board, then you might want to consider getting thermal adhesive.  It is like a little red glue that you put on the components that you reflow first on to the board so that when you flip it over to reflow the other side, the parts don't fall off.