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 $238.94   11.761%   $267.0417   $267.04   $267.05   $267.00  
  Purchase #2 $178.93   13.439%   $202.9764   $202.98   $203.00   $203.00  
  Purchase #3 $71.65   8.612%   $77.8205   $77.82   $77.80   $77.80  
  Purchase #4 $18.53   13.342%   $21.0023   $21.00   $21.00   $21.00  
  Purchase #5 $170.27   10.727%   $188.5349   $188.53   $188.55   $188.50  
  Purchase #6 $102.32   6.218%   $108.6823   $108.68   $108.70   $108.70  
  Purchase #7 $167.52   5.210%   $176.2478   $176.25   $176.25   $176.20  
  7 purchase totals = $948.16       $1,042.3059   $1,042.30   $1,042.35   $1,042.20