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 $147.59   12.333%   $165.7923   $165.79   $165.80   $165.80  
  Purchase #2 $96.74   5.774%   $102.3258   $102.33   $102.35   $102.30  
  Purchase #3 $40.76   9.207%   $44.5128   $44.51   $44.50   $44.50  
  Purchase #4 $243.74   3.177%   $251.4836   $251.48   $251.50   $251.50  
  Purchase #5 $204.50   9.683%   $224.3017   $224.30   $224.30   $224.30  
  Purchase #6 $93.51   8.777%   $101.7174   $101.72   $101.70   $101.70  
  Purchase #7 $98.94   4.666%   $103.5565   $103.56   $103.55   $103.60  
  7 purchase totals = $925.78       $993.6901   $993.69   $993.70   $993.70