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 $108.90   11.232%   $121.1316   $121.13   $121.15   $121.10  
  Purchase #2 $105.59   11.828%   $118.0792   $118.08   $118.10   $118.10  
  Purchase #3 $117.91   4.646%   $123.3881   $123.39   $123.40   $123.40  
  Purchase #4 $227.72   13.647%   $258.7969   $258.80   $258.80   $258.80  
  Purchase #5 $155.61   6.760%   $166.1292   $166.13   $166.15   $166.10  
  Purchase #6 $147.68   8.753%   $160.6064   $160.61   $160.60   $160.60  
  Purchase #7 $135.46   5.652%   $143.1162   $143.12   $143.10   $143.10  
  7 purchase totals = $998.87       $1,091.2476   $1,091.26   $1,091.30   $1,091.20