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 $37.51   13.811%   $42.6905   $42.69   $42.70   $42.70  
  Purchase #2 $13.60   11.346%   $15.1431   $15.14   $15.15   $15.10  
  Purchase #3 $68.79   10.161%   $75.7798   $75.78   $75.80   $75.80  
  Purchase #4 $132.94   11.290%   $147.9489   $147.95   $147.95   $147.90  
  Purchase #5 $189.99   10.607%   $210.1422   $210.14   $210.15   $210.10  
  Purchase #6 $37.66   9.194%   $41.1225   $41.12   $41.10   $41.10  
  Purchase #7 $5.89   7.437%   $6.328   $6.33   $6.35   $6.30  
  7 purchase totals = $486.38       $539.1550   $539.15   $539.20   $539.00