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 $85.37   5.410%   $89.9885   $89.99   $90.00   $90.00  
  Purchase #2 $36.35   12.778%   $40.9948   $40.99   $41.00   $41.00  
  Purchase #3 $227.98   12.543%   $256.5755   $256.58   $256.60   $256.60  
  Purchase #4 $225.28   13.783%   $256.3303   $256.33   $256.35   $256.30  
  Purchase #5 $104.84   5.344%   $110.4426   $110.44   $110.45   $110.40  
  Purchase #6 $39.70   12.639%   $44.7177   $44.72   $44.70   $44.70  
  Purchase #7 $217.46   6.570%   $231.7471   $231.75   $231.75   $231.70  
  7 purchase totals = $936.98       $1,030.7965   $1,030.80   $1,030.85   $1,030.70