You're right that grammatically "siehe" is imperative singular of "sehen", but it's commonly used this way even in formal contexts, without "bitte". The same is true for the alternative "Vergleiche" / "Vgl.". It's not wrong to use "bitte" together with "siehe" or "vgl.", but it's completely uncommon. When formulating a complete sentence, as in Geziefer's answer, using "bitte" would be ok, but it isn't required.
"Siehe" or "vgl." is mostly used in writing, not in spoken language, and usually only to address an undetermined group of persons. When talking or addressing a certain person in a letter / E-Mail, i would prefer a formulated version, for example:
"Mehr zu dieser Frage finden Sie auf Seite 34 des Handbuches."
"Vergleiche das Handbuch, Seite 34."
Regarding the verb, using simply "sehen" isn't the best choice. "Nachsehen", "nachlesen", "nachschlagen", (ein Thema) "vertiefen" are nice alternatives.
The rule of thumb may be:
- If addressing a certain person, don't use "siehe" or "vergleiche" but formulate a complete sentence.
- If you don't address a certain person, using "siehe" of "vgl." (without "bitte") is completely fine. Some people may object that "vergleiche" is too abstract (especially in academic circles), and it would be better to clarify the cross-reference ("why should i compare XY to this?"). But i wouldn't care about this if except when writing a thesis in human sciences.