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 $220.33   12.681%   $248.27   $248.27   $248.25   $248.30  
  Purchase #2 $65.57   9.434%   $71.7559   $71.76   $71.75   $71.80  
  Purchase #3 $175.79   11.494%   $195.9953   $196.00   $196.00   $196.00  
  Purchase #4 $172.35   12.524%   $193.9351   $193.94   $193.95   $193.90  
  Purchase #5 $203.52   12.148%   $228.2436   $228.24   $228.25   $228.20  
  Purchase #6 $20.44   10.790%   $22.6455   $22.65   $22.65   $22.60  
  Purchase #7 $8.78   11.604%   $9.7988   $9.80   $9.80   $9.80  
  7 purchase totals = $866.78       $970.6442   $970.66   $970.65   $970.60