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 $33.82   12.399%   $38.0133   $38.01   $38.00   $38.00  
  Purchase #2 $191.37   6.776%   $204.3372   $204.34   $204.35   $204.30  
  Purchase #3 $157.93   10.757%   $174.9185   $174.92   $174.90   $174.90  
  Purchase #4 $109.74   7.137%   $117.5721   $117.57   $117.55   $117.60  
  Purchase #5 $246.46   8.603%   $267.663   $267.66   $267.65   $267.70  
  Purchase #6 $104.76   8.595%   $113.7641   $113.76   $113.75   $113.80  
  Purchase #7 $62.57   6.716%   $66.7722   $66.77   $66.75   $66.80  
  7 purchase totals = $906.65       $983.0404   $983.03   $982.95   $983.10