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 $68.94   12.488%   $77.5492   $77.55   $77.55   $77.50  
  Purchase #2 $186.54   8.692%   $202.7541   $202.75   $202.75   $202.80  
  Purchase #3 $29.65   12.290%   $33.294   $33.29   $33.30   $33.30  
  Purchase #4 $21.73   4.118%   $22.6248   $22.62   $22.60   $22.60  
  Purchase #5 $151.62   8.231%   $164.0998   $164.10   $164.10   $164.10  
  Purchase #6 $246.35   11.540%   $274.7788   $274.78   $274.80   $274.80  
  Purchase #7 $201.31   4.567%   $210.5038   $210.50   $210.50   $210.50  
  7 purchase totals = $906.14       $985.6045   $985.59   $985.60   $985.60