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 $69.81   12.176%   $78.3101   $78.31   $78.30   $78.30  
  Purchase #2 $137.75   8.534%   $149.5056   $149.51   $149.50   $149.50  
  Purchase #3 $24.63   3.803%   $25.5667   $25.57   $25.55   $25.60  
  Purchase #4 $105.84   9.499%   $115.8937   $115.89   $115.90   $115.90  
  Purchase #5 $41.61   6.275%   $44.221   $44.22   $44.20   $44.20  
  Purchase #6 $184.78   9.402%   $202.153   $202.15   $202.15   $202.20  
  Purchase #7 $223.59   7.334%   $239.9881   $239.99   $240.00   $240.00  
  7 purchase totals = $788.01       $855.6382   $855.64   $855.60   $855.70