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 $40.36   4.472%   $42.1649   $42.16   $42.15   $42.20  
  Purchase #2 $24.60   4.200%   $25.6332   $25.63   $25.65   $25.60  
  Purchase #3 $99.79   6.197%   $105.974   $105.97   $105.95   $106.00  
  Purchase #4 $157.37   13.612%   $178.7912   $178.79   $178.80   $178.80  
  Purchase #5 $16.30   9.132%   $17.7885   $17.79   $17.80   $17.80  
  Purchase #6 $15.53   10.262%   $17.1237   $17.12   $17.10   $17.10  
  Purchase #7 $47.77   13.581%   $54.2576   $54.26   $54.25   $54.30  
  7 purchase totals = $401.72       $441.7331   $441.72   $441.70   $441.80