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 $89.33   10.171%   $98.4158   $98.42   $98.40   $98.40  
  Purchase #2 $81.58   7.285%   $87.5231   $87.52   $87.50   $87.50  
  Purchase #3 $184.34   13.650%   $209.5024   $209.50   $209.50   $209.50  
  Purchase #4 $82.70   8.808%   $89.9842   $89.98   $90.00   $90.00  
  Purchase #5 $181.47   3.482%   $187.7888   $187.79   $187.80   $187.80  
  Purchase #6 $202.35   13.309%   $229.2808   $229.28   $229.30   $229.30  
  Purchase #7 $17.55   12.158%   $19.6837   $19.68   $19.70   $19.70  
  7 purchase totals = $839.32       $922.1788   $922.17   $922.20   $922.20