Is it bad if I held solder with my mouth?

It's a question of bio-availablity, which is facilitated by acids and lead that is more "absorbable" when it is in certain compounds. Certainly Lead accumulates in tissues but even sub toxic doses would take years to accumulate. The Romans used lead based wine mugs because the acidic wine would dissolve the lead which apparently made the wine more palatable (sweeter). No one is willing to do the taste test of that one, that certainly would be unethical for a tester to ask someone to do that. It took the Roman years to get to toxic levels.

As long as you weren't chewing down on the lead and having a good floss with it, it is very likely that very little transferred into your system. I know this because I have myself had blood tests. The Dr. told me it was unlikely that they could detect anything as the lead tends to be gathered into tissues and does not tend to be available in blood. the next option was a biopsy which is more dangerous that any possible lead given predicted exposure.

TLDR; don't worry about having done this, but don't continue.

Heavy metals don't really flush from your system, they accumulate- so its probably a really bad idea. I try to use helping hands or blutac or piles or desk clutter to hold my bits and pieces in place. I have also seen people stab components into old erasers and other bizarre things.

Good solder is only 37% lead, but I still wouldn't be doing that. As well as outright lead poisoning, there could be subtle effects below the poisoning threshold. If it drops your IQ by 10 points would that be acceptable?

Edit: As @Blrfl says, loss of mental acuity is a common symptom of lead poisoning. It's not clear that there is a threshold below which lead is safe, especially for kids.

Really, "there is no safe level of blood lead in children," said Christopher Portier, of the Centers for Disease Control and Prevention. He heads the agency's environmental health programs.

Also, if you don't care about your body, please think of the electronics- your viscous organics-laden spittle and drool won't do much for the integrity of the joints.