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 $182.97   12.466%   $205.779   $205.78   $205.80   $205.80  
  Purchase #2 $60.41   3.858%   $62.7406   $62.74   $62.75   $62.70  
  Purchase #3 $117.69   3.608%   $121.9363   $121.94   $121.95   $121.90  
  Purchase #4 $204.67   13.567%   $232.4376   $232.44   $232.45   $232.40  
  Purchase #5 $113.48   8.288%   $122.8852   $122.89   $122.90   $122.90  
  Purchase #6 $56.64   9.464%   $62.0004   $62.00   $62.00   $62.00  
  Purchase #7 $134.42   12.652%   $151.4268   $151.43   $151.45   $151.40  
  7 purchase totals = $870.28       $959.2059   $959.22   $959.30   $959.10