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 $168.78   9.218%   $184.3381   $184.34   $184.35   $184.30  
  Purchase #2 $140.95   5.731%   $149.0278   $149.03   $149.05   $149.00  
  Purchase #3 $140.37   12.475%   $157.8812   $157.88   $157.90   $157.90  
  Purchase #4 $239.46   11.486%   $266.9644   $266.96   $266.95   $267.00  
  Purchase #5 $71.40   13.804%   $81.2561   $81.26   $81.25   $81.30  
  Purchase #6 $47.85   9.257%   $52.2795   $52.28   $52.30   $52.30  
  Purchase #7 $33.35   5.205%   $35.0859   $35.09   $35.10   $35.10  
  7 purchase totals = $842.16       $926.8330   $926.84   $926.90   $926.90