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 $85.78   10.463%   $94.7552   $94.76   $94.75   $94.80  
  Purchase #2 $106.59   7.805%   $114.9093   $114.91   $114.90   $114.90  
  Purchase #3 $37.65   8.792%   $40.9602   $40.96   $40.95   $41.00  
  Purchase #4 $203.68   13.342%   $230.855   $230.86   $230.85   $230.90  
  Purchase #5 $42.66   6.723%   $45.528   $45.53   $45.55   $45.50  
  Purchase #6 $146.38   8.368%   $158.6291   $158.63   $158.65   $158.60  
  Purchase #7 $177.68   8.822%   $193.3549   $193.35   $193.35   $193.40  
  7 purchase totals = $800.42       $878.9917   $879.00   $879.00   $879.10