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 $242.61   11.290%   $270.0007   $270.00   $270.00   $270.00  
  Purchase #2 $86.34   10.495%   $95.4014   $95.40   $95.40   $95.40  
  Purchase #3 $45.92   12.522%   $51.6701   $51.67   $51.65   $51.70  
  Purchase #4 $27.27   12.778%   $30.7546   $30.75   $30.75   $30.80  
  Purchase #5 $168.32   12.546%   $189.4374   $189.44   $189.45   $189.40  
  Purchase #6 $173.40   3.108%   $178.7893   $178.79   $178.80   $178.80  
  Purchase #7 $219.37   9.548%   $240.3154   $240.32   $240.30   $240.30  
  7 purchase totals = $963.23       $1,056.3689   $1,056.37   $1,056.35   $1,056.40