Working in Landscape Maintenance in the Northwest US with lots and lots of rain, well, rain and wet couldn't come between my crews and their routes. We used hydro static gas mowers for traction more than anything. Mowers that aren't hydro static will spin their wheels and dig up/ruin the grass. A reel mower would be less intrusive to the environment being light and propelled by human feet. If the grass is not too long, the soil not so mushy that you leave deep imprints and most importantly the blades are super sharp there should be no problem mowing in the rain. Another rule to always be using is to change your pattern of mowing EVERY SINGLE TIME you mow!!! Otherwise, mowing in wet conditions will quickly make grooves that are not condusive for a healthy, even lawn. Mow a different pattern each time you mow even in great, dry weather. Try using an angle versus parallel to the house or walk! A continuous circle, a diagonal! I'd easily take a day off mowing if I was not committed to a route. Always mow once or more per week. Especially with a reel mower that doesn't bag clippings. The more often you mow the less you have to remove from the top of your lawn and the finer cut will be your clippings. Finer chopped clippings will decompose quickly. I would fertilize with fertilizer that incorporates bacteria necessary for decomposing these clippings. Dr. Earth Lawn Fertilizer is one example. Also, make SURE you aerate with an aerator that pulls 2-3" plugs out of the lawn. Leave the plugs where they are!! They decompose/break up quickly and help to include soil, bacteria, organic matter to the top of the lawn and thatch beginnings.
Keep your grass height NO SHORTER THAN 3". Grass growth slows down at this height and will be more erect/stiffened than new and faster growth from mowing too short. You'll take of less. Weed seeds won't be able to get enough sun to germinate. Less water will evaporate. Most importantly 'cool season' grasses NEED to have at least 3" of topgrowth to FEED its roots by photosynthesis. Fertilizer IS NOT FOOD. Plants make their own food and if you want healthy grass you've got to hold to the 3" minimum!
I have also on super wet days used my gas blower to 'stand' the grass blades up by blowing off the excess water holding the blades down below the blades of my mower. Or after I mow the first pass, then I used the blower and that would pop up the 'MOHAWKS' of grass that didn't get cut. Mowing twice or three times (on smaller lawns) was not abnormal. Gas powered mowers have a 'sucking' or a vacuum action sucking the grass blades UP to be available to be cut. I always trimmed edges first so that my mower could 'vacuum' up the clippings/leaves. Reel mowers don't really do that. If you have a smallish lawn, a reel mower IS best! Just keep those blades super sharp, mow OFTEN (twice per week) to ensure the clippings are small enough to decompose, aerate once per year and find fertilizer that adds thatch decomposing bacteria. Change your 'pattern' each time you mow!!