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 $239.98   4.861%   $251.6454   $251.65   $251.65   $251.60  
  Purchase #2 $30.26   10.414%   $33.4113   $33.41   $33.40   $33.40  
  Purchase #3 $145.27   7.654%   $156.389   $156.39   $156.40   $156.40  
  Purchase #4 $234.82   13.856%   $267.3567   $267.36   $267.35   $267.40  
  Purchase #5 $43.60   5.621%   $46.0508   $46.05   $46.05   $46.10  
  Purchase #6 $126.32   4.274%   $131.7189   $131.72   $131.70   $131.70  
  Purchase #7 $91.37   11.618%   $101.9854   $101.99   $102.00   $102.00  
  7 purchase totals = $911.62       $988.5575   $988.57   $988.55   $988.60