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 $119.56   13.373%   $135.5488   $135.55   $135.55   $135.50  
  Purchase #2 $243.29   5.771%   $257.3303   $257.33   $257.35   $257.30  
  Purchase #3 $83.26   9.742%   $91.3712   $91.37   $91.35   $91.40  
  Purchase #4 $190.49   4.747%   $199.5326   $199.53   $199.55   $199.50  
  Purchase #5 $152.86   10.757%   $169.3032   $169.30   $169.30   $169.30  
  Purchase #6 $247.90   12.712%   $279.413   $279.41   $279.40   $279.40  
  Purchase #7 $119.36   13.246%   $135.1704   $135.17   $135.15   $135.20  
  7 purchase totals = $1,156.72       $1,267.6695   $1,267.66   $1,267.65   $1,267.60