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 $29.99   6.219%   $31.8551   $31.86   $31.85   $31.90  
  Purchase #2 $132.48   3.851%   $137.5818   $137.58   $137.60   $137.60  
  Purchase #3 $122.32   7.307%   $131.2579   $131.26   $131.25   $131.30  
  Purchase #4 $201.73   6.743%   $215.3327   $215.33   $215.35   $215.30  
  Purchase #5 $114.76   13.302%   $130.0254   $130.03   $130.05   $130.00  
  Purchase #6 $45.80   11.218%   $50.9378   $50.94   $50.95   $50.90  
  Purchase #7 $172.27   11.407%   $191.9208   $191.92   $191.90   $191.90  
  7 purchase totals = $819.35       $888.9115   $888.92   $888.95   $888.90