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 $18.54   10.820%   $20.546   $20.55   $20.55   $20.50  
  Purchase #2 $18.84   8.302%   $20.4041   $20.40   $20.40   $20.40  
  Purchase #3 $98.45   8.373%   $106.6932   $106.69   $106.70   $106.70  
  Purchase #4 $98.32   6.118%   $104.3352   $104.34   $104.35   $104.30  
  Purchase #5 $210.74   5.404%   $222.1284   $222.13   $222.15   $222.10  
  Purchase #6 $103.78   8.342%   $112.4373   $112.44   $112.45   $112.40  
  Purchase #7 $95.26   13.750%   $108.3583   $108.36   $108.35   $108.40  
  7 purchase totals = $643.93       $694.9025   $694.91   $694.95   $694.80