Yeah, I like to use round numbers for the spawn interval, and make the level height and width integer multiples of a single screen where possible. I even set the number of lemmings so that the percentage requirement is a round number (e.g. if the level is lose-4, it will be 36/40 rather than 46/50).
I generally stick to the grid, even with rough terrain. I like the way I just have to select between a few possible placements for each piece, rather than agonising over finding the perfect pixel. And hatches / goals absolutely
must be on a multiple of 16 (or sometimes 8).
I also like to add decoration outside the playing area. And I always love it when a level makes something that
looks like decorative terrain actually be a vital part of the solution
I'm not great at thinking up solutions with main ideas, so instead, I tend to specialise in resource management levels, to the extent that Simon has referred to them as "Proxima-type": there is a restrictive but balanced skillset, no single intended solution, but finding
any solution is hard. Often, I make levels like this by starting with the terrain (which may come from duplicating an easy level; basing the terrain on something external, like the Repton levels used for "Toccata" and "Finale"; or combining bits of existing levels), solving the level while trying to keep to a low and balanced number of skills, and setting the skillset to a nice-looking pattern of numbers that just covers the solution I used. Usually, trying to "balance" means trying to minimise the usage of the skills that would be most useful for solving the level (often builders and miners), so these levels end up needing interesting tricks like placing a bridge to connect two blocks so a miner can go through both; turning a digger with a blocker to save a builder; climb-bombers to get a lemming into the right place to start building; and so on.
I like levels that look simple and uncluttered. I'm not a fan of NeoLemmix's overload of object types, and probably will never use most of them. The objects that existed in L1 are plenty enough to make interesting levels with.