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 $52.39   8.718%   $56.9574   $56.96   $56.95   $57.00  
  Purchase #2 $88.94   9.778%   $97.6366   $97.64   $97.65   $97.60  
  Purchase #3 $6.32   13.293%   $7.1601   $7.16   $7.15   $7.20  
  Purchase #4 $51.57   13.787%   $58.68   $58.68   $58.70   $58.70  
  Purchase #5 $185.32   10.844%   $205.4161   $205.42   $205.40   $205.40  
  Purchase #6 $96.34   5.683%   $101.815   $101.82   $101.80   $101.80  
  Purchase #7 $57.68   10.426%   $63.6937   $63.69   $63.70   $63.70  
  7 purchase totals = $538.56       $591.3589   $591.37   $591.35   $591.40