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 $177.84   5.640%   $187.8702   $187.87   $187.85   $187.90  
  Purchase #2 $234.89   7.784%   $253.1738   $253.17   $253.15   $253.20  
  Purchase #3 $63.58   10.650%   $70.3513   $70.35   $70.35   $70.40  
  Purchase #4 $6.25   11.773%   $6.9858   $6.99   $7.00   $7.00  
  Purchase #5 $141.77   8.686%   $154.0841   $154.08   $154.10   $154.10  
  Purchase #6 $134.50   8.296%   $145.6581   $145.66   $145.65   $145.70  
  Purchase #7 $159.26   9.178%   $173.8769   $173.88   $173.90   $173.90  
  7 purchase totals = $918.09       $992.0002   $992.00   $992.00   $992.20