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 $212.62   13.332%   $240.9665   $240.97   $240.95   $241.00  
  Purchase #2 $77.54   10.834%   $85.9407   $85.94   $85.95   $85.90  
  Purchase #3 $102.39   12.213%   $114.8949   $114.89   $114.90   $114.90  
  Purchase #4 $64.64   11.803%   $72.2695   $72.27   $72.25   $72.30  
  Purchase #5 $7.77   11.697%   $8.6789   $8.68   $8.70   $8.70  
  Purchase #6 $39.72   5.654%   $41.9658   $41.97   $41.95   $42.00  
  Purchase #7 $233.35   13.251%   $264.2712   $264.27   $264.25   $264.30  
  7 purchase totals = $738.03       $828.9875   $828.99   $828.95   $829.10