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 $81.60   8.298%   $88.3712   $88.37   $88.35   $88.40  
  Purchase #2 $249.74   11.730%   $279.0345   $279.03   $279.05   $279.00  
  Purchase #3 $225.62   9.711%   $247.53   $247.53   $247.55   $247.50  
  Purchase #4 $223.40   10.725%   $247.3597   $247.36   $247.35   $247.40  
  Purchase #5 $54.66   6.775%   $58.3632   $58.36   $58.35   $58.40  
  Purchase #6 $70.29   12.346%   $78.968   $78.97   $78.95   $79.00  
  Purchase #7 $88.56   11.387%   $98.6443   $98.64   $98.65   $98.60  
  7 purchase totals = $993.87       $1,098.2709   $1,098.26   $1,098.25   $1,098.30