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 $18.77   7.118%   $20.106   $20.11   $20.10   $20.10  
  Purchase #2 $8.77   6.239%   $9.3172   $9.32   $9.30   $9.30  
  Purchase #3 $200.91   10.103%   $221.2079   $221.21   $221.20   $221.20  
  Purchase #4 $172.34   10.431%   $190.3168   $190.32   $190.30   $190.30  
  Purchase #5 $93.94   10.163%   $103.4871   $103.49   $103.50   $103.50  
  Purchase #6 $216.83   9.516%   $237.4635   $237.46   $237.45   $237.50  
  Purchase #7 $67.88   10.746%   $75.1744   $75.17   $75.15   $75.20  
  7 purchase totals = $779.44       $857.0729   $857.08   $857.00   $857.10