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 $18.57   8.791%   $20.2025   $20.20   $20.20   $20.20  
  Purchase #2 $39.62   11.290%   $44.0931   $44.09   $44.10   $44.10  
  Purchase #3 $105.97   13.491%   $120.2664   $120.27   $120.25   $120.30  
  Purchase #4 $114.81   12.148%   $128.7571   $128.76   $128.75   $128.80  
  Purchase #5 $203.91   10.586%   $225.4959   $225.50   $225.50   $225.50  
  Purchase #6 $5.25   4.842%   $5.5042   $5.50   $5.50   $5.50  
  Purchase #7 $98.65   11.421%   $109.9168   $109.92   $109.90   $109.90  
  7 purchase totals = $586.78       $654.2360   $654.24   $654.20   $654.30