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 $224.67   11.860%   $251.3159   $251.32   $251.30   $251.30  
  Purchase #2 $186.75   13.820%   $212.5589   $212.56   $212.55   $212.60  
  Purchase #3 $127.99   9.784%   $140.5125   $140.51   $140.50   $140.50  
  Purchase #4 $146.77   13.230%   $166.1877   $166.19   $166.20   $166.20  
  Purchase #5 $159.49   10.573%   $176.3529   $176.35   $176.35   $176.40  
  Purchase #6 $140.48   3.546%   $145.4614   $145.46   $145.45   $145.50  
  Purchase #7 $209.49   10.230%   $230.9208   $230.92   $230.90   $230.90  
  7 purchase totals = $1,195.64       $1,323.3101   $1,323.31   $1,323.25   $1,323.40