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 $76.61   10.488%   $84.6449   $84.64   $84.65   $84.60  
  Purchase #2 $152.95   13.442%   $173.5095   $173.51   $173.50   $173.50  
  Purchase #3 $22.36   11.746%   $24.9864   $24.99   $25.00   $25.00  
  Purchase #4 $129.54   4.780%   $135.732   $135.73   $135.75   $135.70  
  Purchase #5 $76.62   6.618%   $81.6907   $81.69   $81.70   $81.70  
  Purchase #6 $121.74   5.784%   $128.7814   $128.78   $128.80   $128.80  
  Purchase #7 $44.91   9.358%   $49.1127   $49.11   $49.10   $49.10  
  7 purchase totals = $624.73       $678.4576   $678.45   $678.50   $678.40