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 $20.31   12.195%   $22.7868   $22.79   $22.80   $22.80  
  Purchase #2 $7.41   3.437%   $7.6647   $7.66   $7.65   $7.70  
  Purchase #3 $12.72   4.393%   $13.2788   $13.28   $13.30   $13.30  
  Purchase #4 $15.77   10.438%   $17.4161   $17.42   $17.40   $17.40  
  Purchase #5 $219.26   9.148%   $239.3179   $239.32   $239.30   $239.30  
  Purchase #6 $129.67   6.768%   $138.4461   $138.45   $138.45   $138.40  
  Purchase #7 $112.46   3.207%   $116.0666   $116.07   $116.05   $116.10  
  7 purchase totals = $517.60       $554.9770   $554.99   $554.95   $555.00