Interesting question. Intuitively, I would have thought that chronic stress would uncompromisingly be a detriment to our memory and recall.
An article by Ohira et al (2011) makes note that it is accepted that chronic stress is seen to adversely affect memory, it may not be replicated in other studies:
"However, results related to the effects of chronic job stress
and cognitive functions remain controversial. Although
chronic job stress is generally linked to reduced memory
and learning (Holman and Wall, 2002; Taris and Feij,
2004), a recent epidemiological study with a larger sample
size failed to replicate these effects (Elovainio et al., 2009)."
Other studies may indicate that predictable chronic mild stress can improve hippocampal neurogenesis and memory (Parihar et al., 2011). Your question doesn't describe the level of stress experienced as a variable so broadly, this article might be very relevant:
"Furthermore, enrichments in mood and memory after PCMS were
associated with substantially increased hippocampal neurogenesis ...
The overall stress reaction in an organism can be advantageous for
cognitive functions when it induces moderate increases in
corticosterone levels and leads to adaptation and resistance ... The
PCMS regimen used in this study likely elicited moderate increases in
corticosterone levels, which facilitated adaptive and beneficial
responses in the brain."
It's implied that unpredictable chronic stress would have a detrimental effect on memory. Some of the biological reasons for enhancing memory processes are presented in that above study.
EDIT: Additional Information
Also see - McFadden, L.M., Paris, J.J., Mitzelfelt, M.S., McDonough, S., Frye, C.A. & Matuszewich, L. (2011). Sex-dependent effects of chronic unpredictable stress in the water maze. Physiology & Behavior, 102(3), 266-275. The research conversely highlights that unpredictable stress improves performance on a spatial memory task in female rats.
- Ohira, H., Matsunga, M., Kumra, K., Murakami, H., Osumi., T., Isowa, T., Fukuyama, S., Shinoda, J. & Yamada, J. (2011). Chronic stress modulates neural and cardiovascular responses during reversal learning. Neuroscience, 193, 193-204
- Parihar, VK., Hattiangady, B., Kuruba, R., Shuai, B. & Shetty, AK. (2011). Predictable chronic mild stress improves mood, hippocampal neurogenesis and memory. Molecular Psychiatry, 16, 171-183