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 $226.57   13.232%   $256.5497   $256.55   $256.55   $256.50  
  Purchase #2 $113.39   13.469%   $128.6625   $128.66   $128.65   $128.70  
  Purchase #3 $221.36   11.468%   $246.7456   $246.75   $246.75   $246.70  
  Purchase #4 $86.66   6.463%   $92.2608   $92.26   $92.25   $92.30  
  Purchase #5 $232.34   4.417%   $242.6025   $242.60   $242.60   $242.60  
  Purchase #6 $220.89   10.411%   $243.8869   $243.89   $243.90   $243.90  
  Purchase #7 $152.56   3.427%   $157.7882   $157.79   $157.80   $157.80  
  7 purchase totals = $1,253.77       $1,368.4962   $1,368.50   $1,368.50   $1,368.50