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 $111.78   7.709%   $120.3971   $120.40   $120.40   $120.40  
  Purchase #2 $127.71   13.366%   $144.7797   $144.78   $144.80   $144.80  
  Purchase #3 $68.76   12.550%   $77.3894   $77.39   $77.40   $77.40  
  Purchase #4 $29.98   6.125%   $31.8163   $31.82   $31.80   $31.80  
  Purchase #5 $214.58   11.373%   $238.9842   $238.98   $239.00   $239.00  
  Purchase #6 $17.31   10.528%   $19.1324   $19.13   $19.15   $19.10  
  Purchase #7 $169.30   12.105%   $189.7938   $189.79   $189.80   $189.80  
  7 purchase totals = $739.42       $822.2929   $822.29   $822.35   $822.30