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 $240.77   12.787%   $271.5573   $271.56   $271.55   $271.60  
  Purchase #2 $137.95   13.592%   $156.7002   $156.70   $156.70   $156.70  
  Purchase #3 $36.94   5.611%   $39.0127   $39.01   $39.00   $39.00  
  Purchase #4 $146.38   3.484%   $151.4799   $151.48   $151.50   $151.50  
  Purchase #5 $107.84   11.608%   $120.3581   $120.36   $120.35   $120.40  
  Purchase #6 $110.86   9.584%   $121.4848   $121.48   $121.50   $121.50  
  Purchase #7 $236.30   4.111%   $246.0143   $246.01   $246.00   $246.00  
  7 purchase totals = $1,017.04       $1,106.6073   $1,106.60   $1,106.60   $1,106.70