shifting gears?

i'm not sure where this thread belongs, so i'm just gonna post it here.

when i shift, i shift around every 10 mph...except my 1st.

1st gear: 1-20
2nd gear: 20-30
3rd gear: 30-40
4th gear: 40-50
5th gear: 50 +

but my question is this...

if i'm gonna slow down, should i down shift around to the level or can i just keep going?

for example, if i'm going around 30-35, should i be driving at 4th gear? or is that high? maybe 3rd?

thanks to anyone that can help.