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 $100.94   12.550%   $113.608   $113.61   $113.60   $113.60  
  Purchase #2 $28.69   8.354%   $31.0868   $31.09   $31.10   $31.10  
  Purchase #3 $120.43   12.344%   $135.2959   $135.30   $135.30   $135.30  
  Purchase #4 $116.65   10.620%   $129.0382   $129.04   $129.05   $129.00  
  Purchase #5 $108.28   3.250%   $111.7991   $111.80   $111.80   $111.80  
  Purchase #6 $69.70   7.491%   $74.9212   $74.92   $74.90   $74.90  
  Purchase #7 $45.90   10.382%   $50.6653   $50.67   $50.65   $50.70  
  7 purchase totals = $590.59       $646.4145   $646.43   $646.40   $646.40