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 $81.78   12.455%   $91.9657   $91.97   $91.95   $92.00  
  Purchase #2 $63.84   6.757%   $68.1537   $68.15   $68.15   $68.20  
  Purchase #3 $84.30   3.757%   $87.4672   $87.47   $87.45   $87.50  
  Purchase #4 $53.59   13.222%   $60.6757   $60.68   $60.70   $60.70  
  Purchase #5 $214.27   8.828%   $233.1858   $233.19   $233.20   $233.20  
  Purchase #6 $25.75   6.329%   $27.3797   $27.38   $27.40   $27.40  
  Purchase #7 $10.33   12.347%   $11.6054   $11.61   $11.60   $11.60  
  7 purchase totals = $533.86       $580.4332   $580.45   $580.45   $580.60