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 $240.98   13.269%   $272.9556   $272.96   $272.95   $273.00  
  Purchase #2 $236.76   4.186%   $246.6708   $246.67   $246.65   $246.70  
  Purchase #3 $241.71   9.346%   $264.3002   $264.30   $264.30   $264.30  
  Purchase #4 $226.56   6.177%   $240.5546   $240.55   $240.55   $240.60  
  Purchase #5 $243.59   9.749%   $267.3376   $267.34   $267.35   $267.30  
  Purchase #6 $32.81   3.538%   $33.9708   $33.97   $33.95   $34.00  
  Purchase #7 $62.74   3.275%   $64.7947   $64.79   $64.80   $64.80  
  7 purchase totals = $1,285.15       $1,390.5843   $1,390.58   $1,390.55   $1,390.70