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 $101.78   11.755%   $113.7442   $113.74   $113.75   $113.70  
  Purchase #2 $146.38   10.530%   $161.7938   $161.79   $161.80   $161.80  
  Purchase #3 $186.99   7.715%   $201.4163   $201.42   $201.40   $201.40  
  Purchase #4 $242.38   12.662%   $273.0702   $273.07   $273.05   $273.10  
  Purchase #5 $96.90   9.694%   $106.2935   $106.29   $106.30   $106.30  
  Purchase #6 $134.29   4.760%   $140.6822   $140.68   $140.70   $140.70  
  Purchase #7 $21.57   13.231%   $24.4239   $24.42   $24.40   $24.40  
  7 purchase totals = $930.29       $1,021.4241   $1,021.41   $1,021.40   $1,021.40