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 $26.27   8.786%   $28.5781   $28.58   $28.60   $28.60  
  Purchase #2 $20.35   7.329%   $21.8415   $21.84   $21.85   $21.80  
  Purchase #3 $142.81   5.408%   $150.5332   $150.53   $150.55   $150.50  
  Purchase #4 $12.47   6.350%   $13.2618   $13.26   $13.25   $13.30  
  Purchase #5 $119.37   7.460%   $128.275   $128.28   $128.30   $128.30  
  Purchase #6 $243.95   10.170%   $268.7597   $268.76   $268.75   $268.80  
  Purchase #7 $22.56   9.730%   $24.7551   $24.76   $24.75   $24.80  
  7 purchase totals = $587.78       $636.0044   $636.01   $636.05   $636.10