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 $237.66   12.561%   $267.5125   $267.51   $267.50   $267.50  
  Purchase #2 $5.76   4.165%   $5.9999   $6.00   $6.00   $6.00  
  Purchase #3 $200.58   4.587%   $209.7806   $209.78   $209.80   $209.80  
  Purchase #4 $22.79   7.573%   $24.5159   $24.52   $24.50   $24.50  
  Purchase #5 $36.55   11.284%   $40.6743   $40.67   $40.65   $40.70  
  Purchase #6 $53.87   7.578%   $57.9523   $57.95   $57.95   $58.00  
  Purchase #7 $22.43   8.276%   $24.2863   $24.29   $24.30   $24.30  
  7 purchase totals = $579.64       $630.7218   $630.72   $630.70   $630.80