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 $38.90   3.799%   $40.3778   $40.38   $40.40   $40.40  
  Purchase #2 $133.70   11.500%   $149.0755   $149.08   $149.10   $149.10  
  Purchase #3 $173.94   4.710%   $182.1326   $182.13   $182.15   $182.10  
  Purchase #4 $177.35   3.485%   $183.5306   $183.53   $183.55   $183.50  
  Purchase #5 $224.73   8.224%   $243.2118   $243.21   $243.20   $243.20  
  Purchase #6 $155.40   6.150%   $164.9571   $164.96   $164.95   $165.00  
  Purchase #7 $31.76   9.574%   $34.8007   $34.80   $34.80   $34.80  
  7 purchase totals = $935.78       $998.0861   $998.09   $998.15   $998.10