I think you have to take in account what makes the actual spell in the first place.
The spells from this site, or any site...are just words. Words with some meaning but usually little importance.
What makes the spell is the energy, or will behind it. You don't need words or candles or tools in a spell. You can focus on your intent silently and still cast an effective "spell".
A sigil wouldn't null the spell at all.
While I agree the subconscious can work against the conscious, creating the sigil in the first place still requires focus of desire, and concentrating on the sigil after created is STILL focusing on the desire.
My usual rule of thumb for doing any spell is to breathe deeply and think of the outcomes. If any of the possible outcomes I can logically think of make me feel bad or unsure...I don't do the spell, or do one to help me to find strength/courage/wisdom to over come said situation. That, for me atleast, helps me have the success I do when casting and not have any ill feelings or guilt hinder the flow of energy.