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 $16.87   8.862%   $18.365   $18.37   $18.35   $18.40  
  Purchase #2 $18.64   13.651%   $21.1845   $21.18   $21.20   $21.20  
  Purchase #3 $128.88   12.669%   $145.2078   $145.21   $145.20   $145.20  
  Purchase #4 $190.93   8.386%   $206.9414   $206.94   $206.95   $206.90  
  Purchase #5 $104.69   6.174%   $111.1536   $111.15   $111.15   $111.20  
  Purchase #6 $168.79   7.102%   $180.7775   $180.78   $180.80   $180.80  
  Purchase #7 $35.79   3.186%   $36.9303   $36.93   $36.95   $36.90  
  7 purchase totals = $664.59       $720.5601   $720.56   $720.60   $720.60