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 $18.85   7.826%   $20.3252   $20.33   $20.35   $20.30  
  Purchase #2 $31.58   3.476%   $32.6777   $32.68   $32.70   $32.70  
  Purchase #3 $145.67   9.348%   $159.2872   $159.29   $159.30   $159.30  
  Purchase #4 $78.44   6.799%   $83.7731   $83.77   $83.75   $83.80  
  Purchase #5 $49.65   3.248%   $51.2626   $51.26   $51.25   $51.30  
  Purchase #6 $118.36   12.599%   $133.2722   $133.27   $133.25   $133.30  
  Purchase #7 $168.95   3.673%   $175.1555   $175.16   $175.15   $175.20  
  7 purchase totals = $611.50       $655.7535   $655.76   $655.75   $655.90