Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $249.64   13.198%   $282.5875   $282.59   $282.60   $282.60  
  Purchase #2 $92.38   12.306%   $103.7483   $103.75   $103.75   $103.70  
  Purchase #3 $36.25   13.636%   $41.1931   $41.19   $41.20   $41.20  
  Purchase #4 $131.57   11.545%   $146.7598   $146.76   $146.75   $146.80  
  Purchase #5 $23.69   11.648%   $26.4494   $26.45   $26.45   $26.40  
  Purchase #6 $50.34   4.681%   $52.6964   $52.70   $52.70   $52.70  
  Purchase #7 $171.79   9.181%   $187.562   $187.56   $187.55   $187.60  
  7 purchase totals = $755.66       $840.9965   $841.00   $841.00   $841.00