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 $139.99   3.438%   $144.8029   $144.80   $144.80   $144.80  
  Purchase #2 $169.95   5.295%   $178.9489   $178.95   $178.95   $178.90  
  Purchase #3 $197.50   4.636%   $206.6561   $206.66   $206.65   $206.70  
  Purchase #4 $222.66   11.789%   $248.9094   $248.91   $248.90   $248.90  
  Purchase #5 $159.44   10.489%   $176.1637   $176.16   $176.15   $176.20  
  Purchase #6 $128.95   6.465%   $137.2866   $137.29   $137.30   $137.30  
  Purchase #7 $218.40   13.713%   $248.3492   $248.35   $248.35   $248.30  
  7 purchase totals = $1,236.89       $1,341.1168   $1,341.12   $1,341.10   $1,341.10