When you cast a vote you spend a little bit of your voting mana, but it regenerates slowly over time. I think the best strategy is to keep it above 70-80%. If it reaches 100% it means you are missing out on voting rewards, so it's optimal to have it under 100% at all times.

Using less then 100% of your voting power (HP, note this is different from voting mana) only matters if you have a lot of HP or if you are casting a lot of votes, like when you are following a curation trail. When you use less than 100% of your voting power you will also consume less voting mana, meaning you can cast more votes (and each vote will be worth less) before hitting the 70-80% threshold.

Following a curation trail casts votes automatically for you by following votes cast by curation communities. It helps you keep your voting mana always under 100% while ensuring the votes you cast go to worthwhile posts.