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 $241.84   13.111%   $273.5476   $273.55   $273.55   $273.50  
  Purchase #2 $85.73   9.240%   $93.6515   $93.65   $93.65   $93.70  
  Purchase #3 $119.75   11.111%   $133.0554   $133.06   $133.05   $133.10  
  Purchase #4 $24.79   4.797%   $25.9792   $25.98   $26.00   $26.00  
  Purchase #5 $223.89   12.363%   $251.5695   $251.57   $251.55   $251.60  
  Purchase #6 $167.84   10.222%   $184.9966   $185.00   $185.00   $185.00  
  Purchase #7 $31.25   5.359%   $32.9247   $32.92   $32.90   $32.90  
  7 purchase totals = $895.09       $995.7245   $995.73   $995.70   $995.80