this is why I disagree with DL's approach to subjunctive. It's a complex theme and it's just drip fed to us with no explanation.
Both solle and sollte are subjunctive and either can be used, in spoken deutsch solle is rarely used. You should not think of sollte as a "past tense" even though it is called the preterite form.
Late response :) As a german, the only context i would use "solle" in would be indirect speech, e.g. "Er sagte, ich solle aus dem Weg gehen." or maybe "Es hieß, das Haus solle nächste Woche abgerissen werden." (Word was that the house should be broken down next week.) There could be more, but I believe it is correct to use Konjunktiv II instead of K I in every case (I'm not sure!!). Sadly most people do that, or just ignore the whole wonderful world of Konjunktiv.
I got this link from someone else in some other thread http://www.dartmouth.edu/~german/Grammatik/Subjunctive/KonjunktivII.html
Not sure if I can explain it well, but when referring to the present tense (sollte can also be past...), "Ich soll" has a stronger sense of 'obligation' than "Ich sollte." Perhaps in "ich soll," someone told you to, but "ich sollte" is more like... you feel like its a better idea to.
Talking about the past, you can also use "ich sollte" to mean 'i should have'