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 $153.57   3.766%   $159.3534   $159.35   $159.35   $159.40  
  Purchase #2 $232.39   9.652%   $254.8203   $254.82   $254.80   $254.80  
  Purchase #3 $118.65   12.643%   $133.6509   $133.65   $133.65   $133.70  
  Purchase #4 $178.92   8.187%   $193.5682   $193.57   $193.55   $193.60  
  Purchase #5 $59.80   6.635%   $63.7677   $63.77   $63.75   $63.80  
  Purchase #6 $13.96   6.476%   $14.864   $14.86   $14.85   $14.90  
  Purchase #7 $95.66   3.857%   $99.3496   $99.35   $99.35   $99.30  
  7 purchase totals = $852.95       $919.3741   $919.37   $919.30   $919.50