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 $239.42   13.510%   $271.7656   $271.77   $271.75   $271.80  
  Purchase #2 $86.62   6.212%   $92.0008   $92.00   $92.00   $92.00  
  Purchase #3 $152.57   12.241%   $171.2461   $171.25   $171.25   $171.20  
  Purchase #4 $197.37   3.110%   $203.5082   $203.51   $203.50   $203.50  
  Purchase #5 $76.74   5.418%   $80.8978   $80.90   $80.90   $80.90  
  Purchase #6 $46.77   3.271%   $48.2998   $48.30   $48.30   $48.30  
  Purchase #7 $55.31   5.460%   $58.3299   $58.33   $58.35   $58.30  
  7 purchase totals = $854.80       $926.0482   $926.06   $926.05   $926.00