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 $70.33   7.412%   $75.5429   $75.54   $75.55   $75.50  
  Purchase #2 $51.89   3.180%   $53.5401   $53.54   $53.55   $53.50  
  Purchase #3 $245.50   5.321%   $258.5631   $258.56   $258.55   $258.60  
  Purchase #4 $208.77   11.307%   $232.3756   $232.38   $232.40   $232.40  
  Purchase #5 $83.38   6.192%   $88.5429   $88.54   $88.55   $88.50  
  Purchase #6 $235.75   6.822%   $251.8329   $251.83   $251.85   $251.80  
  Purchase #7 $57.56   5.357%   $60.6435   $60.64   $60.65   $60.60  
  7 purchase totals = $953.18       $1,021.0410   $1,021.03   $1,021.10   $1,020.90