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 $19.31   11.644%   $21.5585   $21.56   $21.55   $21.60  
  Purchase #2 $201.40   12.104%   $225.7775   $225.78   $225.80   $225.80  
  Purchase #3 $196.25   13.137%   $222.0314   $222.03   $222.05   $222.00  
  Purchase #4 $97.34   4.418%   $101.6405   $101.64   $101.65   $101.60  
  Purchase #5 $87.87   8.146%   $95.0279   $95.03   $95.05   $95.00  
  Purchase #6 $167.70   9.309%   $183.3112   $183.31   $183.30   $183.30  
  Purchase #7 $59.93   6.640%   $63.9094   $63.91   $63.90   $63.90  
  7 purchase totals = $829.80       $913.2564   $913.26   $913.30   $913.20