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 $77.49   6.823%   $82.7771   $82.78   $82.80   $82.80  
  Purchase #2 $197.72   9.583%   $216.6675   $216.67   $216.65   $216.70  
  Purchase #3 $16.78   13.641%   $19.069   $19.07   $19.05   $19.10  
  Purchase #4 $94.69   8.776%   $103   $103.00   $103.00   $103.00  
  Purchase #5 $247.54   8.503%   $268.5883   $268.59   $268.60   $268.60  
  Purchase #6 $33.36   11.826%   $37.3052   $37.31   $37.30   $37.30  
  Purchase #7 $61.57   7.144%   $65.9686   $65.97   $65.95   $66.00  
  7 purchase totals = $729.15       $793.3757   $793.39   $793.35   $793.50