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 $75.55   4.474%   $78.9301   $78.93   $78.95   $78.90  
  Purchase #2 $84.74   6.260%   $90.0447   $90.04   $90.05   $90.00  
  Purchase #3 $224.48   11.608%   $250.5376   $250.54   $250.55   $250.50  
  Purchase #4 $14.99   13.742%   $17.0499   $17.05   $17.05   $17.00  
  Purchase #5 $119.31   8.533%   $129.4907   $129.49   $129.50   $129.50  
  Purchase #6 $148.64   7.173%   $159.3019   $159.30   $159.30   $159.30  
  Purchase #7 $90.92   4.133%   $94.6777   $94.68   $94.70   $94.70  
  7 purchase totals = $758.63       $820.0326   $820.03   $820.10   $819.90