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 $164.57   12.135%   $184.5406   $184.54   $184.55   $184.50  
  Purchase #2 $49.93   4.203%   $52.0286   $52.03   $52.05   $52.00  
  Purchase #3 $203.89   10.357%   $225.0069   $225.01   $225.00   $225.00  
  Purchase #4 $145.80   4.211%   $151.9396   $151.94   $151.95   $151.90  
  Purchase #5 $59.33   9.595%   $65.0227   $65.02   $65.00   $65.00  
  Purchase #6 $58.43   11.343%   $65.0577   $65.06   $65.05   $65.10  
  Purchase #7 $24.29   10.292%   $26.7899   $26.79   $26.80   $26.80  
  7 purchase totals = $706.24       $770.3860   $770.39   $770.40   $770.30