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 $238.99   13.200%   $270.5367   $270.54   $270.55   $270.50  
  Purchase #2 $50.35   5.314%   $53.0256   $53.03   $53.05   $53.00  
  Purchase #3 $156.38   8.288%   $169.3408   $169.34   $169.35   $169.30  
  Purchase #4 $122.41   7.194%   $131.2162   $131.22   $131.20   $131.20  
  Purchase #5 $244.77   5.264%   $257.6547   $257.65   $257.65   $257.70  
  Purchase #6 $65.64   9.292%   $71.7393   $71.74   $71.75   $71.70  
  Purchase #7 $53.35   13.656%   $60.6355   $60.64   $60.65   $60.60  
  7 purchase totals = $931.89       $1,014.1488   $1,014.16   $1,014.20   $1,014.00