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 $250.71   6.496%   $266.9961   $267.00   $267.00   $267.00  
  Purchase #2 $136.52   7.190%   $146.3358   $146.34   $146.35   $146.30  
  Purchase #3 $154.29   6.125%   $163.7403   $163.74   $163.75   $163.70  
  Purchase #4 $199.33   10.142%   $219.546   $219.55   $219.55   $219.50  
  Purchase #5 $249.87   13.347%   $283.2201   $283.22   $283.20   $283.20  
  Purchase #6 $46.54   11.199%   $51.752   $51.75   $51.75   $51.80  
  Purchase #7 $146.57   11.357%   $163.216   $163.22   $163.20   $163.20  
  7 purchase totals = $1,183.83       $1,294.8063   $1,294.82   $1,294.80   $1,294.70