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 $28.37   6.341%   $30.1689   $30.17   $30.15   $30.20  
  Purchase #2 $226.90   4.510%   $237.1332   $237.13   $237.15   $237.10  
  Purchase #3 $23.45   12.706%   $26.4296   $26.43   $26.45   $26.40  
  Purchase #4 $85.65   11.366%   $95.385   $95.39   $95.40   $95.40  
  Purchase #5 $11.31   10.274%   $12.472   $12.47   $12.45   $12.50  
  Purchase #6 $91.96   9.824%   $100.9942   $100.99   $101.00   $101.00  
  Purchase #7 $120.87   13.242%   $136.8756   $136.88   $136.90   $136.90  
  7 purchase totals = $588.51       $639.4585   $639.46   $639.50   $639.50