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 $238.65   8.779%   $259.6011   $259.60   $259.60   $259.60  
  Purchase #2 $200.49   6.217%   $212.9545   $212.95   $212.95   $213.00  
  Purchase #3 $206.81   5.817%   $218.8401   $218.84   $218.85   $218.80  
  Purchase #4 $224.40   10.664%   $248.33   $248.33   $248.35   $248.30  
  Purchase #5 $160.76   10.648%   $177.8777   $177.88   $177.90   $177.90  
  Purchase #6 $57.66   8.257%   $62.421   $62.42   $62.40   $62.40  
  Purchase #7 $123.56   4.293%   $128.8644   $128.86   $128.85   $128.90  
  7 purchase totals = $1,212.33       $1,308.8888   $1,308.88   $1,308.90   $1,308.90