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 $225.81   10.120%   $248.662   $248.66   $248.65   $248.70  
  Purchase #2 $6.83   13.784%   $7.7714   $7.77   $7.75   $7.80  
  Purchase #3 $45.65   9.419%   $49.9498   $49.95   $49.95   $49.90  
  Purchase #4 $226.65   7.739%   $244.1904   $244.19   $244.20   $244.20  
  Purchase #5 $30.76   12.513%   $34.609   $34.61   $34.60   $34.60  
  Purchase #6 $153.50   13.625%   $174.4144   $174.41   $174.40   $174.40  
  Purchase #7 $58.68   9.819%   $64.4418   $64.44   $64.45   $64.40  
  7 purchase totals = $747.88       $824.0388   $824.03   $824.00   $824.00