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 $18.31   10.838%   $20.2944   $20.29   $20.30   $20.30  
  Purchase #2 $182.77   6.370%   $194.4124   $194.41   $194.40   $194.40  
  Purchase #3 $48.91   6.318%   $52.0001   $52.00   $52.00   $52.00  
  Purchase #4 $135.27   10.110%   $148.9458   $148.95   $148.95   $148.90  
  Purchase #5 $92.69   6.120%   $98.3626   $98.36   $98.35   $98.40  
  Purchase #6 $7.56   9.319%   $8.2645   $8.26   $8.25   $8.30  
  Purchase #7 $90.41   13.401%   $102.5258   $102.53   $102.55   $102.50  
  7 purchase totals = $575.92       $624.8056   $624.80   $624.80   $624.80