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 $90.57   10.401%   $99.9902   $99.99   $100.00   $100.00  
  Purchase #2 $157.54   10.753%   $174.4803   $174.48   $174.50   $174.50  
  Purchase #3 $133.39   7.616%   $143.549   $143.55   $143.55   $143.50  
  Purchase #4 $12.80   13.190%   $14.4883   $14.49   $14.50   $14.50  
  Purchase #5 $206.26   9.499%   $225.8526   $225.85   $225.85   $225.90  
  Purchase #6 $13.70   13.691%   $15.5757   $15.58   $15.60   $15.60  
  Purchase #7 $97.39   9.569%   $106.7092   $106.71   $106.70   $106.70  
  7 purchase totals = $711.65       $780.6453   $780.65   $780.70   $780.70