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 $249.47   7.133%   $267.2647   $267.26   $267.25   $267.30  
  Purchase #2 $84.46   4.609%   $88.3528   $88.35   $88.35   $88.40  
  Purchase #3 $174.57   9.835%   $191.739   $191.74   $191.75   $191.70  
  Purchase #4 $147.71   4.568%   $154.4574   $154.46   $154.45   $154.50  
  Purchase #5 $26.49   11.669%   $29.5811   $29.58   $29.60   $29.60  
  Purchase #6 $19.45   4.801%   $20.3838   $20.38   $20.40   $20.40  
  Purchase #7 $243.52   3.307%   $251.5732   $251.57   $251.55   $251.60  
  7 purchase totals = $945.67       $1,003.3520   $1,003.34   $1,003.35   $1,003.50