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 $87.98   3.300%   $90.8833   $90.88   $90.90   $90.90  
  Purchase #2 $17.72   12.153%   $19.8735   $19.87   $19.85   $19.90  
  Purchase #3 $223.41   8.874%   $243.2354   $243.24   $243.25   $243.20  
  Purchase #4 $132.38   3.268%   $136.7062   $136.71   $136.70   $136.70  
  Purchase #5 $60.25   11.245%   $67.0251   $67.03   $67.05   $67.00  
  Purchase #6 $168.29   6.730%   $179.6159   $179.62   $179.60   $179.60  
  Purchase #7 $78.26   3.192%   $80.7581   $80.76   $80.75   $80.80  
  7 purchase totals = $768.29       $818.0975   $818.11   $818.10   $818.10