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 $27.83   13.326%   $31.5386   $31.54   $31.55   $31.50  
  Purchase #2 $136.62   8.132%   $147.7299   $147.73   $147.75   $147.70  
  Purchase #3 $151.90   8.659%   $165.053   $165.05   $165.05   $165.10  
  Purchase #4 $185.97   11.228%   $206.8507   $206.85   $206.85   $206.90  
  Purchase #5 $224.94   10.352%   $248.2258   $248.23   $248.25   $248.20  
  Purchase #6 $177.58   9.802%   $194.9864   $194.99   $195.00   $195.00  
  Purchase #7 $146.95   13.255%   $166.4282   $166.43   $166.45   $166.40  
  7 purchase totals = $1,051.79       $1,160.8126   $1,160.82   $1,160.90   $1,160.80