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 $219.38   13.482%   $248.9568   $248.96   $248.95   $249.00  
  Purchase #2 $246.42   11.738%   $275.3448   $275.34   $275.35   $275.30  
  Purchase #3 $165.56   8.212%   $179.1558   $179.16   $179.15   $179.20  
  Purchase #4 $104.33   7.108%   $111.7458   $111.75   $111.75   $111.70  
  Purchase #5 $234.66   9.541%   $257.0489   $257.05   $257.05   $257.00  
  Purchase #6 $102.79   12.659%   $115.8022   $115.80   $115.80   $115.80  
  Purchase #7 $59.29   13.252%   $67.1471   $67.15   $67.15   $67.10  
  7 purchase totals = $1,132.43       $1,255.2014   $1,255.21   $1,255.20   $1,255.10