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 $25.92   10.761%   $28.7093   $28.71   $28.70   $28.70  
  Purchase #2 $16.86   7.214%   $18.0763   $18.08   $18.10   $18.10  
  Purchase #3 $14.41   11.419%   $16.0555   $16.06   $16.05   $16.10  
  Purchase #4 $101.56   8.513%   $110.2058   $110.21   $110.20   $110.20  
  Purchase #5 $101.47   10.437%   $112.0604   $112.06   $112.05   $112.10  
  Purchase #6 $163.60   8.776%   $177.9575   $177.96   $177.95   $178.00  
  Purchase #7 $22.98   12.796%   $25.9205   $25.92   $25.90   $25.90  
  7 purchase totals = $446.80       $488.9853   $489.00   $488.95   $489.10