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 $18.35   11.337%   $20.4303   $20.43   $20.45   $20.40  
  Purchase #2 $90.74   4.117%   $94.4758   $94.48   $94.50   $94.50  
  Purchase #3 $115.63   3.630%   $119.8274   $119.83   $119.85   $119.80  
  Purchase #4 $219.68   12.210%   $246.5029   $246.50   $246.50   $246.50  
  Purchase #5 $144.32   8.586%   $156.7113   $156.71   $156.70   $156.70  
  Purchase #6 $87.58   11.734%   $97.8566   $97.86   $97.85   $97.90  
  Purchase #7 $142.50   9.775%   $156.4294   $156.43   $156.45   $156.40  
  7 purchase totals = $818.80       $892.2337   $892.24   $892.30   $892.20