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 $52.81   8.475%   $57.2856   $57.29   $57.30   $57.30  
  Purchase #2 $70.78   13.549%   $80.37   $80.37   $80.35   $80.40  
  Purchase #3 $23.87   6.673%   $25.4628   $25.46   $25.45   $25.50  
  Purchase #4 $191.60   7.497%   $205.9643   $205.96   $205.95   $206.00  
  Purchase #5 $86.32   12.851%   $97.413   $97.41   $97.40   $97.40  
  Purchase #6 $144.50   11.292%   $160.8169   $160.82   $160.80   $160.80  
  Purchase #7 $68.58   5.345%   $72.2456   $72.25   $72.25   $72.20  
  7 purchase totals = $638.46       $699.5582   $699.56   $699.50   $699.60