There are different ways of looking at this. You could assess the likelihood of needing to make a claim, for example, a streetlight on a busy road may be struck by a passing vehicle, whereas a metal bench in a park is far less likely to suffer insurable damage. Some policies determine premiums based on your asset values, whereas others have fixed premiums for a range of values, e.g. up to £25k, £25k to 50k etc. Understand the implications on your premium for your own policy and you may wish to inflate or deflate or even remove items entirely.
For items you perceive the hold the greatest likelihood of making a claim, go for full replacement value, as you'll be paying an excess anyway, so seek to gain the maximum benefit from your cover. If your insurer deems the item to be overvalued for a claim, they'll simply pay a lesser amount. There's no penalty for this.