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 $18.55   8.569%   $20.1395   $20.14   $20.15   $20.10  
  Purchase #2 $12.65   7.138%   $13.553   $13.55   $13.55   $13.60  
  Purchase #3 $174.93   4.798%   $183.3231   $183.32   $183.30   $183.30  
  Purchase #4 $169.80   11.633%   $189.5528   $189.55   $189.55   $189.60  
  Purchase #5 $94.54   4.118%   $98.4332   $98.43   $98.45   $98.40  
  Purchase #6 $65.52   6.468%   $69.7578   $69.76   $69.75   $69.80  
  Purchase #7 $86.86   10.634%   $96.0967   $96.10   $96.10   $96.10  
  7 purchase totals = $622.85       $670.8561   $670.85   $670.85   $670.90