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 $35.49   11.194%   $39.4628   $39.46   $39.45   $39.50  
  Purchase #2 $73.53   3.692%   $76.2447   $76.24   $76.25   $76.20  
  Purchase #3 $180.25   8.155%   $194.9494   $194.95   $194.95   $194.90  
  Purchase #4 $30.33   10.469%   $33.5052   $33.51   $33.50   $33.50  
  Purchase #5 $227.51   6.803%   $242.9875   $242.99   $243.00   $243.00  
  Purchase #6 $95.59   3.605%   $99.036   $99.04   $99.05   $99.00  
  Purchase #7 $185.58   13.365%   $210.3828   $210.38   $210.40   $210.40  
  7 purchase totals = $828.28       $896.5684   $896.57   $896.60   $896.50