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 $238.47   6.177%   $253.2003   $253.20   $253.20   $253.20  
  Purchase #2 $48.99   13.646%   $55.6752   $55.68   $55.70   $55.70  
  Purchase #3 $84.72   6.161%   $89.9396   $89.94   $89.95   $89.90  
  Purchase #4 $145.82   7.294%   $156.4561   $156.46   $156.45   $156.50  
  Purchase #5 $82.35   6.283%   $87.5241   $87.52   $87.50   $87.50  
  Purchase #6 $226.57   7.542%   $243.6579   $243.66   $243.65   $243.70  
  Purchase #7 $32.43   6.113%   $34.4124   $34.41   $34.40   $34.40  
  7 purchase totals = $859.35       $920.8656   $920.87   $920.85   $920.90