The rule of enemy placement at the start of a level is that Nintendo never places an enemy on the first screen. If the player is not paying attention at the start of the level, there is no fear of being hit by an enemy.
First enemies in SMB and SMB2 are completely off-screen, as seen here at the start of SMB World 8-1. The player is safe as long as they do not move. In the wide-screen Super Mario Maker, this placement does not work, as the enemies will start moving immediately.
Most levels in SMB3 have no enemies visible on the first screen. One exception is World 2-5, where a Chain Chomp is visible. The player is safe unless they stand long enough that the Chain Chomp breaks free.
Likewise, Super Mario World does not place enemies at the start of a level. In Vanilla Secret 2, two green-shelled Koopa Paratroopas are just off-screen. In the wide-screen Super Mario Maker, they appear on-screen and begin moving immediately at their starting positions.
One level which does have an immediate danger is the Special World Funky, where a Sumo Bro. drops lightning to cover the starting spot with fire.
In World 7-7 of NSMB is one of the game’s rare instances of having an enemy on the first screen. As is the common pattern of an enemy on the first screen, this red-shelled Koopa Troopa is unable to reach a stationary player.
In Super Mario Maker, if you want to avoid enemies being able to hit a non-moving player, you need to place enemies starting in the second screen, no sooner than the fifth tile. In the screenshot above, the red-shelled Koopa Troopa will advance toward a stationary player at the level’s start, but the green-shelled Koopa Troopa will not advance until the screen begins to scroll.
This placement leaves the start of the level more empty than in the older Super Mario titles due to Super Mario Maker using wide-screen. The wide-screen NSMB titles get around this by zooming out as soon as the player begins moving.
For Super Mario Maker, placing enemies within the first screen is probably forgivable, but ensure the player has a reasonable enough amount of time to respond to the enemy’s advance.