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 $7.40   5.519%   $7.8084   $7.81   $7.80   $7.80  
  Purchase #2 $23.57   11.642%   $26.314   $26.31   $26.30   $26.30  
  Purchase #3 $145.48   3.160%   $150.0772   $150.08   $150.10   $150.10  
  Purchase #4 $231.56   13.210%   $262.1491   $262.15   $262.15   $262.10  
  Purchase #5 $97.92   10.446%   $108.1487   $108.15   $108.15   $108.10  
  Purchase #6 $44.87   7.828%   $48.3824   $48.38   $48.40   $48.40  
  Purchase #7 $233.48   10.592%   $258.2102   $258.21   $258.20   $258.20  
  7 purchase totals = $784.28       $861.0900   $861.09   $861.10   $861.00