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 $64.35   9.698%   $70.5907   $70.59   $70.60   $70.60  
  Purchase #2 $155.97   3.192%   $160.9486   $160.95   $160.95   $160.90  
  Purchase #3 $207.56   7.483%   $223.0917   $223.09   $223.10   $223.10  
  Purchase #4 $146.66   10.853%   $162.577   $162.58   $162.60   $162.60  
  Purchase #5 $198.85   5.592%   $209.9697   $209.97   $209.95   $210.00  
  Purchase #6 $95.93   13.405%   $108.7894   $108.79   $108.80   $108.80  
  Purchase #7 $53.52   3.411%   $55.3456   $55.35   $55.35   $55.30  
  7 purchase totals = $922.84       $991.3127   $991.32   $991.35   $991.30