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 $209.66   11.856%   $234.5173   $234.52   $234.50   $234.50  
  Purchase #2 $60.78   8.189%   $65.7573   $65.76   $65.75   $65.80  
  Purchase #3 $74.98   6.227%   $79.649   $79.65   $79.65   $79.60  
  Purchase #4 $135.59   10.609%   $149.9747   $149.97   $149.95   $150.00  
  Purchase #5 $169.71   9.493%   $185.8206   $185.82   $185.80   $185.80  
  Purchase #6 $181.32   5.412%   $191.133   $191.13   $191.15   $191.10  
  Purchase #7 $79.64   7.805%   $85.8559   $85.86   $85.85   $85.90  
  7 purchase totals = $911.68       $992.7078   $992.71   $992.65   $992.70