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 $19.79   10.616%   $21.8909   $21.89   $21.90   $21.90  
  Purchase #2 $20.89   5.498%   $22.0385   $22.04   $22.05   $22.00  
  Purchase #3 $88.67   10.182%   $97.6984   $97.70   $97.70   $97.70  
  Purchase #4 $119.30   13.554%   $135.4699   $135.47   $135.45   $135.50  
  Purchase #5 $142.38   5.641%   $150.4117   $150.41   $150.40   $150.40  
  Purchase #6 $75.63   6.867%   $80.8235   $80.82   $80.80   $80.80  
  Purchase #7 $125.94   4.362%   $131.4335   $131.43   $131.45   $131.40  
  7 purchase totals = $592.60       $639.7664   $639.76   $639.75   $639.70