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 $26.62   11.731%   $29.7428   $29.74   $29.75   $29.70  
  Purchase #2 $143.59   6.449%   $152.8501   $152.85   $152.85   $152.90  
  Purchase #3 $183.97   9.266%   $201.0167   $201.02   $201.00   $201.00  
  Purchase #4 $146.46   10.454%   $161.7709   $161.77   $161.75   $161.80  
  Purchase #5 $149.63   8.105%   $161.7575   $161.76   $161.75   $161.80  
  Purchase #6 $84.68   8.174%   $91.6017   $91.60   $91.60   $91.60  
  Purchase #7 $25.95   8.137%   $28.0616   $28.06   $28.05   $28.10  
  7 purchase totals = $760.90       $826.8013   $826.80   $826.75   $826.90