It is generally advised to stop medications that may increase the risk of anaphylaxis before any procedure that may induce anaphylaxis. Such procedures include a drug challenge or an oral food challenge. The medication should be stopped for enough period of time to allow for 5 half lives to pass after the last dose.
For example, the elimination half-life of oral atenolol is approximately 6 to 7 hours. Atenolol should not be detectable after 5 half lives, 5 x 7 hours = 35 hours (1.45 days).
From the EAACI guidelines:
Concomitant medications and pretreatments
Before beginning any desensitization procedure, patients should be in a stable clinical condition (e.g., FEV1 ≥ 70% of the normal value for asthmatics, controlled cardiac insufficiency). Any concomitant medication used for treating underlying diseases must be continued. However, drugs such as beta-blockers, which can interfere with the treatment of a severe hypersensitivity reaction, should be discontinued whenever possible. Indications should be strictly observed, e.g., sudden withdrawal of beta-blocker therapy in patients with arrhythmia may be life threatening.
Desensitization to culprit drugs should generally not be performed in patients at increased risk because of a co-morbidity, such as those with uncontrolled asthma (FEV1 < 70% of their normal value), hemodynamically unstable ones, or those with uncontrolled cardiac diseases. In patients treated with beta-blockers and subjects who have experienced severe anaphylaxis, as well as in patients with hepatic, renal, or other diseases, in whom exposure might provoke a potentially harmful complication, desensitization should also only be considered after a careful individual risk/benefit evaluation. Desensitization is absolutely contraindicated in patients who have experienced severe, life-threatening immunocytotoxic reactions, vasculitis, or bullous skin diseases like SJS/TEN, and DIHS/DRESS.
I did not find guidelines on ACEI inhibitors.
General considerations on rapid desensitization for drug hypersensitivity – a consensus statement. Allergy, 2010.
Immunotherapy with ocular beta blocker. AAAAI Ask the Expert 2015.