Personally I wouldn't worry about polishing sprites until you're satisfied that the basic engine works but I would make the effort to make sure that my placeholder or prototype sprites are approximately the same shape and colour as what you plan on eventually using. Otherwise you're very likely to run into bugs and design problems later on (for example, don't use a block when the final sprite is going to be a non geometric shape like a fish or a star.)
I prefer not to use the editor included in Construct, I make all the images in paint.net or photoshop, save them to disk and update them when I decide some polish is needed in case a change I made in the graphics causes some kind of problem and I need to revert back to an earlier version of the sprite to figure out what caused the issue. Don't get me wrong, the editor is perfectly good but an external program gives me more control over masks and transparency.
Prototyping before you bother polishing also makes sure you don't spend hours polishing and perfecting some lovely tiling textures before you notice that you need some additional sprites that you never accounted for and won't fit without scrapping a lot of the work and having to go back to the drawing board. Or worse still, that the sprites just flat out don't work for what you had in mind.
Most games, even the massive budget triple A titles look like blocky crap right up until the last few weeks before release when the developers are sure that everything works the way it should.