For little stuff like 一 being faintly curved, I definitely wouldn’t worry. That’s going to vary through slight differences in handwriting/font.
What I would recommend paying attention to is line length and proportion, because there are some kanji that look REALLY similar and you don’t want to accidentally write the wrong thing; for example, line length makes the difference between writing 土 and 士.
(Even on a computer screen those are tricky! 
)
And of course, make sure not to forget strokes! Because otherwise you wind up writing 刀 instead of 分 and your students give you very confused looks.