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 $169.70   5.170%   $178.4735   $178.47   $178.45   $178.50  
  Purchase #2 $73.96   10.701%   $81.8745   $81.87   $81.85   $81.90  
  Purchase #3 $6.81   13.429%   $7.7245   $7.72   $7.70   $7.70  
  Purchase #4 $218.80   11.197%   $243.299   $243.30   $243.30   $243.30  
  Purchase #5 $9.74   10.166%   $10.7302   $10.73   $10.75   $10.70  
  Purchase #6 $99.40   7.478%   $106.8331   $106.83   $106.85   $106.80  
  Purchase #7 $38.36   6.447%   $40.8331   $40.83   $40.85   $40.80  
  7 purchase totals = $616.77       $669.7679   $669.75   $669.75   $669.70