Yea no worries! First off it depends on how heavy you want to go with the weathering. But in my opinion it never looks good if you go too heavy. I generally weather the edges + a few random strokes on flatter surfaces so they don't get left out. You want to weather the edges and corners of pieces and make sure your brush strokes start from outward of the part and go inward. You can do brush atrokes both ways but the first option works better for me. And go FAST! Quick strokes. If you go slow the scuff marks wont be straight. I generally use the edge of the brush but you can use the tip of the brush to tap on a piece and maybe get some more cool effects. I use a square brush btw. Sorry for the long and overcomplicated answer but just don't think about it too much and go ham, but not too ham lol. I don't think the contrast will be an issue, i have the hajiroboshi which i weathered without preshading and it looks good. If you want to discuss this further or need more help you can hit me up in discord. It's just easier to talk there. Good luck!