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 $6.50   4.846%   $6.815   $6.82   $6.80   $6.80  
  Purchase #2 $71.74   10.229%   $79.0783   $79.08   $79.10   $79.10  
  Purchase #3 $132.95   5.103%   $139.7344   $139.73   $139.75   $139.70  
  Purchase #4 $228.32   6.476%   $243.106   $243.11   $243.10   $243.10  
  Purchase #5 $106.64   3.680%   $110.5644   $110.56   $110.55   $110.60  
  Purchase #6 $249.55   11.317%   $277.7916   $277.79   $277.80   $277.80  
  Purchase #7 $29.34   12.357%   $32.9655   $32.97   $32.95   $33.00  
  7 purchase totals = $825.04       $890.0552   $890.06   $890.05   $890.10