Re: can other things be substituted for dH2O to prepare reagents

While there would be certain cases of particular reagents and tests where 
it would not be critical for the reagents to be prepared in distilled 
water, I would not presume to know which they might be, and therefore, 
I must answer that, "No it would not be 'safe' to substitute tap water."

I would also add that using tap water might lead to a contention that any 
evidential conclusions drawn from the use of such reagents might be brought 
under suspicion by the claim that the result of the test came from 
impurities in the tap water.

Distilled water and deionized water are not the same.  In distilling the 
water, it is ideally separated from all other materials whether they are 
ionic such as salts and minerals or non-ionic such as many organic 
compounds (sugar, urea, alcohol, etc.)which are soluble in water.  By 
boiling it in the distillation, the water is also sterilized.

Deionized water is not boiled but passed through some material which 
removes only the ions which come from ionic substances.  The non-ionic 
sustances are not removed from the water and any bacteria may be still 
present.  So for work that must leave no doubt, deionized water would not 
be a good substitute for distilled water.