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 $118.74   10.834%   $131.6043   $131.60   $131.60   $131.60  
  Purchase #2 $224.74   10.682%   $248.7467   $248.75   $248.75   $248.70  
  Purchase #3 $112.64   12.160%   $126.337   $126.34   $126.35   $126.30  
  Purchase #4 $99.68   12.203%   $111.844   $111.84   $111.85   $111.80  
  Purchase #5 $236.94   4.475%   $247.5431   $247.54   $247.55   $247.50  
  Purchase #6 $93.97   7.823%   $101.3213   $101.32   $101.30   $101.30  
  Purchase #7 $32.73   5.846%   $34.6434   $34.64   $34.65   $34.60  
  7 purchase totals = $919.44       $1,002.0398   $1,002.03   $1,002.05   $1,001.80