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 $18.63   10.305%   $20.5498   $20.55   $20.55   $20.50  
  Purchase #2 $5.57   3.727%   $5.7776   $5.78   $5.80   $5.80  
  Purchase #3 $149.77   12.591%   $168.6275   $168.63   $168.65   $168.60  
  Purchase #4 $10.89   8.677%   $11.8349   $11.83   $11.85   $11.80  
  Purchase #5 $139.26   12.606%   $156.8151   $156.82   $156.80   $156.80  
  Purchase #6 $76.51   9.782%   $83.9942   $83.99   $84.00   $84.00  
  Purchase #7 $165.98   11.282%   $184.7059   $184.71   $184.70   $184.70  
  7 purchase totals = $566.61       $632.3050   $632.31   $632.35   $632.20