It is important to look at as many aspects of the subject property as possible; including activity in the neighborhood, time on the market, reasons for selling (some listing agents will actually come right out and divulge this), price range in which the property falls into, amount owed on the property (at least what you can assume from the tax records), condition of the property, etc.
In short, I think it's reasonable if the situation dictates it. Another school of thought that some agents and buyers are subscribing to is to make the offer and see how the seller responds. If you have no reason to suspect that another offer is in the making, then it may be worthwhile to use that approach - with appropriate justification for the offer, of course. The seller could accept your offer, counter, or refuse it altogether (most skilled listing agents would recommend they counter if the offer is unacceptable).
Danny Gallant, Realtor
The Gallant Group @ Keller Williams Realty